/ Motoring

Car fuel economy claims are miles from the truth

Car wheel

We already know that ‘official’ car mpg figures are rarely possible in the real world – our tests show that. But choosing larger alloy wheels could see your fuel bills increase even more.

As part of the Which? Car team, I get more emails about fuel economy than anything else. They inevitably start by mentioning the official EU test figure, as quoted by the car manufacturer, then explaining how their car can’t get within 20 miles per gallon (mpg) of that number.

That’s why we think our mpg tests are more realistic. They include cold starts and motorway driving – both absent from the official EU cycle.

However, official figures could be even further from the truth – depending on the wheel-size of the car you buy. Opt for big alloys (as many style-conscious buyers do) and your fuel economy could drop significantly.

Different wheels, different mpg

At the moment, Toyota and its sister-brand Lexus are the only companies to quote separate economy and CO2 emissions figures depending on wheel size.

For example, buy a Lexus IS 300h in entry-level SE spec and it comes with 16-inch wheels. Quoted economy is 65.7mpg with CO2 emissions of 99g/km (grams of carbon dioxide per kilometre driven). However, stretch your budget to the Luxury model and you’ll also need to stretch your fuel allowance; its 17-inch alloys reduce efficiency to 64.2mpg (103g/km CO2).

At the top of the Lexus IS range is the F Sport version. This has exactly the same engine and electric motor, but with 18-inch rims its claimed economy is 60.1mpg (109g/km CO2). That’s an increase of nearly 10% over the SE model – almost entirely because of those larger wheels and tyres.

To put that in perspective, a driver covering 12,000 miles a year would spend £107 more on fuel for the F Sport than the IS 300h SE. Plus, annual car tax (VED) costs £20 – rather than being free.

Same car, different wheels

Now let’s look at one of the IS’s rivals. The BMW 3 series is the UK’s bestselling large car, yet its maker quotes identical fuel economy and CO2 emissions for all models, whether with 17, 18 or 19-inch alloys.

I believe Toyota and Lexus deserve credit for being upfront about fuel economy figures. It’s time other car companies followed suit.

Can you match your car's mpg claims?

No (69%, 784 Votes)

Yes (18%, 205 Votes)

I don't know (13%, 153 Votes)

Total Voters: 1,150

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Comments
Member

It is well known that EU fuel consumption figures do not represent real-life consumption, and are not intended to. They are precisely controlled tests to compare different cars on an equal basis, and presumably to provide a basis for emmission taxation.

The way to get real-life consumption is from drivers reported information – and particularly, I suppose, from fleet operators. Honest John at the Daily Telegraph publishes these. So do Which? and other motoring publications.

Even then, your own consumption will depend on how you drive and your journey pattern, so it would be impossible to give a consumption figure that would apply to all drivers – only a range would make any sense.

Member

For emmission, read emission!
Dear Which? When can you introduce editing of posts please? Even if it involves deleting your offending post and resubmitting it.

Member

Please don’t worry about typos Malcolm – if you are concerned report your comment and I’ll fix the typo. Thanks

Member
Colin Samson says:
15 September 2013

Ha Ha! “Emmission” stuck out like a sore thumb to me too. I’m glad that I’m not alone in finding such errors to be irritating. The spell checker usually shows that a word is misspelled when these posts are being typed, as the word “Emmission is highlighted as I write this now; so there should be no excuse.

“TV’s” is another error showing up on Which? reviews. There should be NO apostrophe as the abbreviation is plural (more than one television); it should spell as “TVs” or arguably “TVs.” with a full stop to show it is an abbreviation of “Televisions”. Unfortunately the daily press is usually littered with spelling & grammatical errors so it is no wonder that standards elsewhere are plummeting.

Member

Hi Colin, Malcolm’s referring to the error in his own comment (sorry Malcolm!) However, if you ever spot a typo in something we’ve written please do tell us. Sometimes mistakes slip through, but we do like our copy to be error free. TV’s is an annoying one! Now, back on the topic of car mpg claims…

Member

We could do with an easier way of reporting errors in the introductions to topics than using a Web-based form and putting in name, email address, subject and message.

Incidentally, the energy campaign logo has KWh rather than kWh. I’ve seen kW/h in an earlier Conversation. 🙁

Cheers

Member

petty Dare I say

Member

But accurate. We should preserve correct English – it has a purpose. Two things (well two of the many things actually) that irritate me are the misuse of apostrophes and beginning spoken sentences with “so”. Sorry to deviate from the topic.

Member

If I was working in a hole in the ground, I would like to know whether the hole would consume 1 man day or 1 man/day . There is a difference.
In scientific reports, it is important to get the units correct. Wrong units cause confusion and make it look like you don’t understand what you are saying.
Apparently our new street lights will save several kW per year. On that basis, perhaps the contractor’s new lorries will save several horse power per week.

Member

Malcolm,
Of course you are quite correct manufacturers figures will always be compiled in absolutely ideal conditions to give the best result possible. After all they want to sell cars. The car buyer has very little chance of matching these consumption figures, but because all manufacturers do it the numbers still offer valuable comparison information.

And lets face it no one drives the same or is as diligent about the cars condition so accurate figures for consumption even if independently derived would still at best only offer a comparison.
So there cannot be any real solution to this situation.
However most of us will realize these manufacturers figures are “optimistic”, but we can still make an informed buying decision based on comparison.

Not much to debate really is there?

Member

Chris, I think it would have helped if Which’s introduction had spelled out exactly how EU figures are derived and that they are not created to represent real life, only as comparative data. Instead it rather suggests they are misleading. | wish that Which? would not employ the sensationalist (tabloid) approach quite so much – it should be objective.

Member

MalcolmR,

Completely agree.

Member

Hello Malcolm and Chris, we’ve debated mpg claims on Conversation quite a few times now, which means we are now simplifying the argument for why we think our tests are more accurate. Here’s our original, more detailed Conversation: https://conversation.which.co.uk/transport-travel/cars-mpg-claims-tested-car-miles-per-gallon-fuel-economy-petrol/

What Tim wants to debate here is specifically the issue of wheel size and mpg.

You can also read a more detailed summary of how we test mpg in our guide, and why we think the EC tests don’t represent real-world driving: http://www.which.co.uk/cars/choosing-a-car/how-we-test-cars/how-we-test-mpg/

Member

I read somewhere that the official figures are rigged to be higher than real life, because a manufacturer will rig the car in favour of a better result, Whether its removing (the now non existent ) spare wheel, to switching off things like air con, disconnecting the alternator ( as why would they need to charge the battery) etc so what they end up testing doesn’t resemble real life at all.

The tests should be carried out on the exact spec and condition a car is likely to do be used in the real world and if anything should be done with a large weight in the boot too

Member

The test is done on a rolling road, so I don’t think weight would have much effect. Drag is definitely excluded since it is not moving through the air. As I understand it the tests are very specific, so unlikely the manufacturer could cheat by disconnecting the alternator. I would think running without aircon would be sensible as long as all cars were tested the same way.
However, these are comparative tests. Perhaps the adverts and literature should make it quite clear that these are not representative of real-life performance. The only way to get this is to refer to the sources above.

Member

Not sure they are done on a rolling road as that would eliminate air resistance.

I’ve read a similar, or was it the same, story. If true manufacturers remove every bit of excess weight, remove the drive to anything that might absorb power, remove door mirrors and even tape over gaps in the bodywork.

Member

Must have been the same story as taping over the gaps in the bodywork rings a bell.

Member

The tests are done on a rolling road. Taping over the bodywork gaps would have no effect. The tests and caveats are described on the following link:
http://www.dft.gov.uk/vca//fcb/exhaust-emissions-testing.asp

Member

Where?

When tests are conducted in a laboratory on a rolling road an electrical fan must be used to simulate air resistance otherwise the tests would be producing even worse results than they do now, especially at higher speeds.

http://www.telegraph.co.uk/motoring/green-motoring/9241054/Fuel-economy-why-your-car-wont-match-the-official-mpg.html

Member

Air resistance is not part of the test. These are laboratory tests for the specific purpose of producing comparative results, not real-life figures, and have never been purporting to do otherwise. For what you might achieve in practice you need real data from cars on real roads, doing real journeys driven by real drivers – and even then there will be a range within which your car with you driving might fall. Which publishes these, as does Honest John.

Member

Air resistance has to be part of the test otherwise the test is useless. The fact that the manufacturers figures can’t be replicated in real life but are supposed to be realistic is the whole point of this conversation although some makers are still claiming that they are realistic and can even be exceeded by careful drivers!

Member

Phil, the whole point is the tests are not realistic, nor claimed to be. I don’t see that the intro to this conversation suggests otherwise. There are moves to improve the test. If you want realistic results then use accumulated drivers reports for a good guide.

Member

It is essential that cars are tested under standard conditions for fuel economy figures to be comparable. To overcome cheating by manufacturers, the cars to be tested should be purchased in the same way that Which? does when carrying out tests of products, and not using cars chosen for tests by the manufacturer.

Member

Late one night I observed a policeman filling his car with diesel. After I had offered him a lift to the local nick, he told me that he was getting 25mpg on average. My last tank full delivered a verifiable 46mpg. We are both supposed to get an average of 56mpg from our cars. Scalded cats apart, the manufacturers’ claims are of no use to anyone; that’s why the latest Which initiative, among others, is so good. It now needs to be comprehensive, officially recognised and mandatory information for any new car spec.

Member

I pay attention to the fuel consumption gauge on my car to help me drive in a reasonably economical way. Even if it is inaccurate, it gives me a useful indication of how cold starts, crawling in rush hour traffic and driving at 70 mph on motorways all use more fuel than longer journeys on reasonably quiet roads out of built-up areas. At the moment, I’m averaging over 60 mpg, but only because I am able to avoid doing many short journeys.

Member

Whilst manufacturers have made good progress with increasing the efficiency of their engines, they are constantly tweaking other aspects to improve MPG figures – eg airflow, lowering the ride height, taller gearing, stop/start etc. Most of these tweaks have little effect on real life fuel economy. For example, with higher gearing, you need to change to a lower gear more often to compensate for variable driving conditions (traffic, bends, junctions, hills, speed limits etc). It’s only constant steady speed driving that will benefit from higher gearing and the scope for that is ever decreasing.

Member

I’m very glad that car manufacturers are constantly tweaking their designs to improve fuel economy.

All the tweaks have given us cars that will easily deliver 50-70 mpg, despite the fact that many of us are driving more aggressively than in earlier years. If you do not like changing gears, there are good automatic transmissions that will do the job without greatly affecting fuel consumption, very different from the early automatic transmission systems using inefficient torque convertors.

Member

That’s the problem – people are not getting such MPG figures in their every day driving. Figures reported in by drivers proves that these claims by car makers are not being achieved. You have completely misunderstood what people are saying. Do some brim to brim testing to find out what you are really getting.

Member

AQ – I am NOT refuting the fact that manufacturers are being dishonest about fuel economy. I think this is dishonest and disgraceful. My point is that the cumulative effect of car design changes have greatly improved actual fuel economy over the years.

I will try to remember to measure my fuel consumption over a few thousand miles, but on the basis of one fill-up, I am achieving nearly 60 mpg at present, with my diesel car. I don’t expect this to be maintained during the winter months or if I start to do a lot of short journeys.

My fuel economy on journeys I make regularly can change by 25% depending on how I am driving and the traffic on the roads.

Member

wavechange /AQ, if you are suggesting that when manufacturers quote EU test figures for fuel consumption they are being dishonest then you are being unfair. They are not claims for what you will get, but report the results of these “laboratory” tests. These tests are done for comparative purposes only and are not intended to represent real life. I posted a link above the goes into some detail on these tests.

Out of interest, I went to Devon on a mixture of rural roads and motorways, with the usual slow traffic at holiday time, in my 2.2 diesel Espace – not a light car. My on-board consumption meter showed 43.5 mpg – just on the extra urban EU figure. And when I returned and added a lot of local driving it dropped to 38.3 – well above the mixed figure. I always fill to a full tank and the consumption calculated from this matches the computer.

Member

Malcolm R – Neither of us were accusing car makers of being dishonest. However, given the specifics of the prescribed EU testing methodology, manufacturers are exploiting the potential to achieve better and better results in such tests – and they have a legal obligation to publicise these test results in their advertising material. Whilst a good many of us know that these figures are unlikely to be achieved on the road, there are probably a much greater number of people who will “trust” this information when making their car buying decisions. Whilst we could criticise the test methodology, because it is so unrealistic, the difficulty is prescribing an alternative which would have so many variables that comparability would be impossible – hence the “laboratory” approach. Perhaps manufacturers should put a disclaimer in their publicity “believing EU inspired statistics can damage your wealth”.

Member

Malcolm and AQ

When I bought a new car last year I looked at the published fuel consumption and read the small print in the brochure explaining that “The driving style, road and traffic conditions, environmental influences and vehicle condition can in practice lead to consumption figures which may differ from those calculated with this standard”. I asked the sales rep what fuel economy I could expect and he quoted the published information, without the disclaimer. That was an experienced sales rep at a VW main dealer.

If the alternative is to have the EU test figure or something more realistic, I think we can guess which most consumers would prefer.

There are so many cases where products and services are not as good as they appear. For example, my ISP used to advertise the broadband service I use as ‘up to 24 Mbps’, and I have never managed to better 7 Mbps. Now that they have dropped the ‘up to’ figure, I am much happier. If VW had published a fuel economy figure of 50mpg, I would be very happy indeed because I can achieve that or better throughout the year.

I’m prepared to retract my claim that the manufacturers are being dishonest on the basis that the small print exists, but maintain that using these figures in marketing is despicable, because they know well that they are not typical of what the motorist can expect to achieve.

Thank goodness that Which? and others publish fuel economy figures that are more realistic.

Member

wavechange – I agree with your view on their use in marketing and sales. It would be difficult, though, to devise a standard test that gives truly realistic data. However, realistic data exists, as we have said. Maybe some of this is authoritative (carefully collected over a range of scenarios) perhaps from fleet operators. If so, I wonder if there is a way such data should be included in advertisements and literature. I’m a bit dubious.
As I have suggested elsewhere the government should help educate consumers deal with the sales and marketing hype exuded by lots of industries and services that seek to mislead us. Tax advertisements to provide the funding. Perhaps this could then educate everyone to consult sources of real-life fuel consumption before visiting the car sales emporium.

Member

You mentioned taxing advertising when we were discussing unhealthy soft drinks, Malcolm, and I can see plenty of potential for raising revenue for education. In this case it could help consumers deal with manufacturers’ hype.

As a scientist who has spent years trying to present information in a clear and unbiased way, the deceit and misrepresentation that is prevalent in advertising really annoys me, but I have long given up hoping for the situation to improve.

Member
Johno Shaft says:
14 September 2013

please explain why bigger wheels mean less mpg….

I would’ve thought it would be the other way round.

for me the main issue is tyres. compare mpg on a motorway with winter tyres vs eco tyres, unbelievable

Member

It’s not specifically the size of the wheels, it’s the overall size of wheel and tyre. When bigger wheels are fitted, it’s usually to take lower profile tyres (to sharpen the handling, but it makes the ride harder). So it’s the circumference right round the tyre that needs to be compared. Normally, there would be very little difference between standard wheels / standard tyres and bigger wheels / lower profile tyres, but there can be occasions when the latter results in a smaller overall circumference, which results in slightly shorter overall gearing (MPH per 1000 revs) and thus the fuel economy – but it is very marginal.

Member

It is far more complicated than that … .

If you look at the range of standard vs. low profile tyres available for the Lexus IS300, you will see that standard 205/55R16 tyres come with a wide range of Energy Label ratings – typically “C” or even “B” ratings. The low profile 255/35R18 tyres fitted to the rear of the Sport model only come in “E” or “F” ratings. In terms of economy, the effect of fitting these less energy-efficient tyres is similar to fitting winter tyres to a normal production vehicle.

Why? In theory, wide low-profile tyres should be more economical than standard profile tyres as there is less deformation of the tyre. However, there is more material, the tyres tend to be heavier and the rubber composition of the tyre may be more geared towards grip than economy, all factors that increase rolling resistance.

In practice, it is probably a recognition by manufacturers that people who buy low profile tyres for looks simply don’t care about fuel economy.

Top marks to Toyota for pointing out the difference, but another Convo opportunity missed to point out the importance of checking the tyre energy label and the tyres’ significance in fuel economy?

… Simply put, tyre rolling resistance accounts for about 10%-20% of the fuel used, so a small change here (different rubber, profile, tread pattern, etc.) can have a noticeable effect on consumption.

Member

Em – yes I know there are lots of other factors concerning tyres that will have an impact on fuel economy. However, I was replying specifically to the comment made on the effect of bigger wheels, particularly in relation to diameter.

Member

if you are buying a top of the range car – you have more money than sense and thus it is obvious you are not really concerned about the cost of fuel or economy

Member

Andi, in principle I agree that by choosing a large-engined heavy car you are not putting economical fuel consumption top of your list. But I disagree that they have more money than sense – as if only by choosing economy can you be regarded as sensible. You could argue that flying abroad for holidays is not sensible (fuel used) for example – we all have the freedom to use our disposable income as we choose! -:)

Member

small car man here – my motorcycle is my statement
my byeline: I’ll stop driving when petrol is as expensive as beer

Member

Andi, at £20 a gallon I think you make a good point!

Member

in my bar that would be more of £40 to the Gallon – so I will be driving for a while yet I think

Member
Jim Simpson says:
14 September 2013

This 13 reg is my 3rd 1.8 Prius : the first two averaged 65.0mpg over 20K, using “Eco” mode usually but switching to “Power” for overtaking when necessary. The current one is following the same profile. “Eco” is more than adequate in town and for motorways and gives a smooth and quiet performance. This mpg equates pretty well with Consumer Reports’ (USA) mpg when converted from US gallons. Pretty early you can adapt your driving style and I usually outperform most cars from the stoplights while maintaining or slightly exceeding the limit. Most car reviewers seem to drive impetuously and so get poor mileage.

Member
Gerard Phelan says:
14 September 2013

It is very difficult to get repeatable “real world” fuel figures, because the “real world” is so variable. Some 35 years ago my employer, an oil company, was trying to validate US results that a new engine oil blend improved the mpg by x%. So 100 brand new cars were bought, 10 each of different models across the ranges. Each group of 10 cars were driven in convoy by experienced drivers, along routes that were the classic mix of motorway, urban, rural, flat and hilly. The order was fixed such that car #3 was always third and car #9 was always ninth, in case the mattered. Odd cars received engine oil A and even cars engine oil B taken from tanks just marked A and B. No-one in the UK knew whether oil A or B was supposed to be the better. The cars were fuelled from a single petrol tanker, so that the same batch of petrol applied throughout the trials. High precision fuel meters and odometers had been added to allow reliable petrol usage and distance travelled data to be logged daily for each car for the few weeks the trials ran.

My role as a fresh faced postgraduate fresh from studying mathematical modelling was as part of the team analysing the results, looking for the step change in mpg we were told we should find. The big problem was the variability of the data, which reflected the complexity of driving in convoys in the “real world”. In towns they got snarled up in traffic, some were held up by traffic lights whilst others escaped into the distance, sometimes they lost the car in front or behind (this is long before mobile phones or sat navs) so some cars would go faster or slower to allow the convoy to reform. Occasionally drivers would forget themselves and overtake others in the convoy, so getting out of their designated position.

Overall a massive variability of data, even before we tried to identify the improvement we were expecting to see. Therefore although as a driver myself I am sympathetic to the desire to see published “real world” mpg figures, I saw just how hard it would be to obtain anything that would be repeatable.

Member

I can certainly relate to research being a bit frustrating at times, Gerard. 🙁

I presume that most cars now have a fuel economy gauge, so most drivers should be aware of the many factors that can affect their fuel consumption, and hopefully realise that providing realistic estimates is not a precise science. The problem is that far more people achieve less than the published mpg than those who manage to exceed it. If lower figures were published there would be fewer dissatisfied motorists and some very happy ones.

It seems daft that you are probably more likely to get a measure of fuel economy by asking friends with the same car than by studying the manufacturer’s data.

Member

Every car I’ve owned over 30+years has returned something resembling the MPG for mixed driving conditions – until I got a jaguar XF 3.0 diesel. I struggled to get 30MPG on a car that is supposed to return 40+ MPG with mixed driving. I complained to Jaguar & they were useless. I took it the the dealer & they filled the tank to the brim, took it for a spin & claimed they got 40+MPG. When I insisted they must have fudged the results & questioned them firmly, they owned up to switching off the air con etc and taking it down the A3 at 55 MPH, and of course staring on a hot engine as I’d just dropped it off. I complained to the top & even the MD of Jag UK didn’t give a stuff.

Member

There is no such thing as a single ‘real world’ fuel consumption figure for a car but rather several such figures with a very big difference between them! One of the major factors in the fuel consumption being achieved is the way the car is driven with the actual road/weather conditions for the journey running close behind. The figures published by the manufacturers are achieved under well defined test conditions, they should be used simply to compare car A with car B and not regarded as indicative of actual ‘real world’ performance.

As is so happens, my car usually returns a consumption figure not too far from the published value during the warm summer months but noticeably lower in the winter. On a recent journey from Birmingham to west of Ipswich I was surprised to see that I had actually bettered the published figure – and I had an appointment to make so was not hanging about!

Member

There seems to be one lesson we can all take away from this Convo; never under-estimate the benefit of an extended (24 hour) test drive. Since nearly all cars now have a trip computer, it is possible to get an accurate real-world estimate of fuel consumption from a typical day’s usage.

A good example of where a car simply didn’t suit my journey pattern or deliver anything like the manufacturer’s stated mpg was the Toyota Prius. If I lived in a city, then great. But although it was very economical around town, the petrol hybrid system wasn’t of any benefit on motorway journeys, where the Prius was overweight (all those batteries) and underpowered.

Re. another Convo: attempting to accelerate a Prius out of danger at 70 mph going up hill in the fast lane is not a good option. A conventional Diesel-engined car turned out to be just as economical for me – and “safer”.

Member

Up until I had my company car I have always bought second hand cars, in order Mini 1L 1983, 1300 Metro 1985, Rover 216 Si 1997, Volvo 740 2L Estate (petrol no turbo) and in all of those cars I could readily exceed the official MPG without even trying too hard – sometimes by more than double the official figure – and without driving like an old dear. I then had a Volvo V50 2005 2L diesel and this, just met the figures, albeit only when I was being careful. Now I have an 61 plate Astra Sports Tourer, which supposedly should do about 67mpg. The best I ever had when I emptied a tank on a motorway and did 62mph all day constant, was 62mpg. If even constant speed at most efficient revs (90 KPH and 100KPH equate to approx 56mph and 62mph and are the tested limits for Europe because of common speed limits there) can’t match the official numbers, then there is something seriously wrong with the official test.

Member

Just read the TestLab feature (Oct 2013) I notice all/most vehicles are diesel which unless the user drives some 10K per year, or at least has regular travel, at speed, over some 10/20 miles then they are liable to suffer particulate filter problems, and the maintenance costs will thence cost more than any fuel saving over petrol.
I feel Which has failed to warn peoples of this problem in their Lab feature

Member

The maintenance costs in the article were specific to each car – so they are correct for the diesel engines – with any DPF-related costs included. These costs come from CAP, our data provider, while the servicing costs quoted come from the 2013 Which? Car Survey.

Member

Regarding insurance costs of the Mazda CX5, I see in your Test Lab article you quote £619 as a typical price. However, my wife pays just £149.50 for fully comprehensive from Sainsbury’s Bank, and we don’t live in a particularly low risk area. I read on another website that the collision avoidance radar on the car has led to large reductions in premiums. Maybe your information is a little out-of-date?

Member

Regarding the poll on this topic – “Vote in our Poll Can you match your car’s mpg claims?”.
This presumably refers to the EU figures that by law must be published by the car-makers? Figures that are well known not to claim to be representative of real-life motoring, but not stated explicitly here.
It would be fairer to ask perhaps if the EU should also collate real life data and publish that , at least to give a better indication of performance. Difficult with a brand new model or engine, which is no doubt partly why laboratory testing is necessary.
I am all in favour of real life data – aren’t we all – but we must be careful not to lead people into thinking they are being mislead.

Member
Joe Cash says:
11 November 2013

In yankee doodle land if cars do not meet the mpg stated by the manufacturer they are heavily fined. A well known Korean manufacture had its wallet emptied of millions of dollars because of this “dodgy” dealing.

Member
Flocker says:
5 May 2014

Just a quick question, do they manufacturers include an air resistance factor (remember the drag coefficient in the 70s) in the simulated tests. As someone mentioned above, without this being factored in, there can not be a comparison of the vehicle with other vehicles even where they have the same engine and drive train.

Member

The answer is no they don’t include it for fuel consumption tests – they can’t do this in a “laboratory” type test, in just the same way they cannot effectively simulate hills and bends or other factors that would influence a real life result. However, given two cars with the same laboratory results, the car with the lower drag coefficient is more likely to give better MPG results in real life driving – all other factors being equal. The EU testing methodology is exploited by manufacturers to tweak settings, gearing, ride height, tyre rolling resistance, weight saving etc to stretch the MPG figures in the test as far as they can so that they can quote the results in their publicity – and the more they do this, the less likely anybody will get anywhere close to their figures. Whilst “economy” versions of cars (eg blue motion, greenline etc) will publicise consumption of say 15 to 20% less than the standard car, owners are more likely to achieve results hardly any better than the standard car. Best to use the figures as nothing more than a rough guide.

Member

The DfT point out: “The fuel consumption testing scheme is intended to give potential car buyers comparative information about the relative fuel consumption of different models in standard tests.” They are not intended to represent real life, but to allow some comparison between different cars. Tests must be standardised for this comparison to make any sense. Individuals will get widely differing results anyway in practice, depending on their journey types, geography, driving style etc. I think the best you can do is to look at published average real life data (such as Honest John and Which?) to get an idea of what you might get near, and treat the EU figures for what they are -comparative and relative only. Moves are afoot apparently to make them more related to normal motoring but use.

Member
Flocker says:
6 May 2014

If the drag factor is not included, then how has the statement ‘to allow comparisons between different vehicle types and different manufacturers’ never been challenged as surely this would have succeeded no matter how big the motor industry is.

Member

Which? has recognised that published fuel economy figures are not very useful. What consumers need is figures that are closer to reality. It would be better to underestimate rather than overestimate fuel economy so that the average driver can achieve or exceed the published figure.

Most car journeys start off with a cold engine, yet the EU tests are performed on engines at full operating temperature. If I recall, Which? does one of their two tests starting off with a cold engine. Perhaps it would be useful to do both with a cold engine.

I acknowledge the need to carry out tests on a rolling road, so that air resistance is ignored. I’m sure it would be possible to adjust the fuel economy figures to allow for how streamlined different vehicles are.

I would like to see all fuel economy tests carried out independently and not by manufacturers.

Member

“EU tests are performed on engines at full operating temperature.” Not so – only the extra-urban cycle, that is continued after the urban, uses an initially hot engine. However, tests are done in high ambients. Dispel the myth that the EU tests represent real life- they don’t and are not claimed to. Just an attempt at comparing vehicles in a relative way. Use other data, as above, to get an idea of real life mpg (l/km).
“Urban cycle
The urban test cycle is carried out in a laboratory at an ambient temperature of 20°C to 30°C on a rolling road from a cold start where the engine has not run for several hours. The cycle consists of a series of accelerations, steady speeds, decelerations and idling. The maximum speed is 31 mph (50 km/h). The average speed 12 mph (19 km/h) and the distance covered is 2.5 miles (4 km).”

Member

Thanks Malcolm. I stand corrected. I see the EU as part of the problem here.

Many of us are aware that the EU fuel economy figures are not representative of real life. That does not stop car dealers using these figures to sell cars. As I reported earlier, an experienced sales rep tried that one on me and he received a lecture about dishonest selling.

Member
Flocker says:
6 May 2014

I have reply from a certain manufacturer that air tests/drag should and is taken into consideration this particular manufacturer. Anyone have a definitive test procedure to conform that drag is or should be taken into consideration.

As far as testing the particular vehicle is concerned there is no reason at all that the test cannot include a simulation of hills of different gradients. The test will certainly have a load applied to the drive wheels to simulate the rolling resistance of the vehicle under test on a level road, and they can simply increase the load to simulate the effect of gravity on the vehicle mass on inclines. It calculable. No need to take into account inertia as in free wheel mots engines shut off fuel flow.

This drag factor is the most important input particularly if its not taken into consideration. Need to know definitively that it is or isn’t.

Member

I agree, Flocker. Even if the simulation is not perfect we could still have figures that are attainable.

One point I disagree about. Even when fuel is not being injected, the engine acts as a brake and thus affects fuel consumption. The way that I can sometimes achieve fuel economy figures similar to the published values is to allow the car to run down long gentle slopes out of gear so that there is no braking effect of the engine. The car remains in control and I don’t inconvenience other motorists.

Member
Flocker says:
7 May 2014

Yes it will affect miles per gallon because the vehicle is moving but your not actually using fuel as you coast/idle down the gradient. On an injection engine in overrun (using the engine as a brake) fuel is effectively shut off so taking the car out of gear shouldnt make any difference to fuel consumption. If you have instant mpg indication, look at the reading it may show off scale say 99.9mpg and when your idling you may see gallons per hour.

But the test could easily provide a more informed guestimate of real world.

So anyone confirm that drag is supposed to be taken into consideration during the tests, anyone from which? This is to me the most crucial input to the loading in real world and if not included makes the official figures useless as a comparison other than comparing engine/gearbox/drive chain between cars under the specific mass load of the vehicle model being tested..

Member

The NEDC (New European Driving cycle) was devised when cars were less powerful and so acceleration was gentler than now. So the cycle needs updating. There is a report produced by the Joint Research Centre (JRC) that supports EU policy making that you may find interesting:
http://publications.jrc.ec.europa.eu/repository/bitstream/111111111/22474/1/co2_report_jrc_format_final2.pdf
The current cycle appears, on average, to underestimate fuel consumption by around 10-15% for petrol, and 12-20% for diesel compared with “in use” driving. For drag (including air resistance) it seems the dynamometer settings that simulate this are based in ideal data that underestimates its effect. The report says:
“5.6 Type-Approval optimization
Although this is not directly an outcome of the study, this is an important conclusion from relevant work that should be re-iterated. Type-approval tests of fuel consumption are conducted on chassis dynamometer using resistance settings provided by the manufacturer. These settings are derived from coast-down vehicle tests. It appears that resistance of actual vehicles measured by independent test centres are higher than the ones submitted by the manufacturers for the type-approval tests. There are several reasons why this can be happening, i.e. manufacturers test vehicles in ideal conditions (tarmac condition, weather, vehicle run-in, configuration such as
tyre dimensions, trained drivers to perform the test, etc.). Unfortunately, type-approval resistance settings are confidential. Using of real vehicle resistances instead of type-approval resistances has been shown to lead to fuel consumption increases of up to 17%. This is even beyond the in-use over type-approval fuel consumption ratio developed in this report. As a minimum impact this means that maybe the NEDC is not a bad (underpowered) cycle to report fuel
consumption but that maybe the actual test is an idealistic one. It can be recommended that vehicle resistance settings become public together with the type-approval fuel consumption value, so that independent authorities can check both whether these represent reality and whether the type-approval test has been conducted as required.”
I would suggest that it is the test rules and conditions that need strengthening, both to reflect the performance of modern vehicles and to avoid the loopholes that can, and will, be exploited by all manufacturers.

Member
Flocker says:
7 May 2014

Thanks for that, I suspect someone got paid handsomely for that lot.

NEDC does state that drag/air resistance is included from a look up table.

Member
Flocker says:
11 May 2014

It is interesting that comparing a car sold in the EU with the same car sold in the US, the US actually provide an estimated mpg. The estimated mpg is nearer the real world figure. I did the comparison by comparing the exact same Eco boost engine (petrol as Diesel option wasn’t available) and the weight was the same within a 20kg. With the old and now antiquated test being adhered to and vehicle manufacturers clearly wanting defending its use and probably reluctant to move to a more realistic results.

I’m just having a discussion with a manufacturer who I won’t name, but having changed from a 2008 140psi Euro 4 engine saloon car, to a 2014 163psi Euro 5 engine all-wheel drive crossover as they call them (which by the way I’m very impressed with) from the same manufacturer, the official figures amazingly are comparable in fact the Euro 5 engine gives a slightly better urban and combined figure the extra urban is within 2 mpg. I thought it not unreasonable that while the official figures are wide of real world, I could expect the new car to return similar real world figures. Having travelled the same commute this past 2 years of 142mile round trip covering almost 36000 of what is better than combined driving of literally 40, 60 and 70 mph the 2008 car returned 46mpg. The new car travelling the same route now for 5600 mile returns 38/39mpg. Ah, one may think that the new engine is not run in, well I’d like to think so too, but 1 year owners of the same car and engine having driven 15000 mile and probably like me a repeatable sustained journey, also get around the 38mpg.

The sales brochure states “The applied standard test procedure enables to compare between different vehicle types and different manufacturers”. (Their grammar by the way). In my change of car I would expect a drop in fuel economy, but the brochure actually showed there wouldn’t be. Clearly the test doesn’t give a comparison between different models within this manufacturer never mind other manufacturers too. I have now queried this with the manufacturer.

Member
Flocker says:
11 May 2014

BTW, the real world combined consumption of the crossover is the same as the urban consumption and the official urban consumption is 39.7mpg. This shouts something very significant to me.

Member
Flocker says:
18 May 2014

Well here’s an update. A reply from the manufacturer say I cannot compare the two cars as one has a bigger engine and has had to meet more stringent emissions control so could reduce mpg, it has bigger wheels (that a joke surely, both are 18”, Hatch has 234/40 SUV 235/50). Here’s the comparison official fuel figures of the hatch in 2008 and 2014. I can’t see where bringing the engine up to Euro stage 5 has increased fuel consumption there.

2008 Euro 4 140 psi 37.2mpg 57.6mpg 47.9 mpg
2014 Euro 5 163psi 46.3mpg 67.3mpg 57.7mpg

So I have replied indicating that they themselves have compared the 2 vehicles by testing them to the same specification to provide a comparison under controlled laboratory conditions between vehicle types and that the type of vehicle should be factored in to the test to enable a comparison to be made, otherwise then figures don’t mean anything.

This particular manufacturer does have a habit of exhibiting a contemptible attitude to owners of their products, but they know we’ll still buy their cars.

So from the above, the manufacturer’s technical liaison team either don’t know the real reason the NEDC test is carried out and what the results are used for, or they think or hope the owners of their vehicles don’t know either.

Member
Flocker says:
18 May 2014

As someone already stated, wish we could edit posts, but excuse the spooling mistooks

234 should be 235 of course

Member
David Harington says:
22 April 2015

It’s not just mpg – electric cars’ range is overstated too.

I bought my Mitsubishi Outlander PHEV on the basis that of the manufacturer’s claim that it would do ‘up to 32 miles’ on electricity only. Actually it only does about 20. I think that overstating by 50% is actually fraudulent.

Member
jdsx says:
23 April 2015

I didn’t buy my Mitsubishi Outlander PHEV on the basis of the manufacturer’s claim – only a fool would do that, surely. I looked at the claims, read a bit about the type of driving I would do, investigated further and made my decision.. I am currently getting 140mpg (excluding electricity costs, of course).

Member
David Ludlow says:
22 April 2015

Whilst it is correct that ‘test’ figures do not reflect real driving I have to say that my previous car, a Mondeo 2.0 litre diesel did better than Ford quoted, achieving 51 mpg, worst 49mpg best 52mpg, and I’m not recognised by friends as a slow driver. Given the difference is size (vehicle and engine, this is close to my current Focus 1.6 diesel that achieves 54 mpg with stop start & other economy toys. Sometimes technology is not as clever as people think!!

Member
Peter Gibson says:
22 April 2015

I am surprised that Which? is making such a fuss about manufacturers’ published mpg figures. Surely we all know that the Euro tests not only bear little relation to real life conditions, but also that actual consumption figures depend to a considerable degree on the individual driving style. The published figures provide a good initial guide to which make or model fares better or worse than another model, but for a purchaser to assume the he/she will achieve the manufacturer’s figure is surely extremely naive. If actual fuel consumption is important to a purchaser, it is sensible to read tests in motoring magazines and Which? Even then, it would be a miracle if any individual was able to match these test results exactly. I rather resent the statement made by Which? that “we each spend an average of £133 more on fuel a year than we expect to”. Many (if not most) of us understand that the manufacturers’ figures are a rough guide, only.

Member

Hi Peter, we believe you’ve forgotten the difference in how much some cars miss their claims compared to others, though?

When our Cars Team tested the Mazda3 Fastback, it had a naturally aspirated 2.0-litre petrol engine. It matched the mpg claims in our test. However, the plug-in hybrid Mitsubishi Outlander PHEV used 120% more fuel than the claims suggest. Therefore, the claimed figures presented to consumers are most certainly not comparable – a Mazda3 2.0-litre petrol owner is being given a far more realistic impression of fuel costs than a Mitsubishi Outlander PHEV buyer.

We’d also disagree about your comments about most knowing about manufacturers’ figures – We think there’s plenty of consumers who do not know this.

Member

Andrew, as far as I am aware the EU figures are not portrayed as mpg (l/km) “claims”. They are results of tests under very specific and consistent test conditions for a particular purpose, which is not to tell you what you will get in practice.

The DfT state “The fuel consumption figures quoted in this guide are obtained under specific test conditions, and therefore may not necessarily be achieved under ‘real life’ driving conditions. A range of factors may influence actual fuel consumption – for example, driving style and behaviour, as well as the environment and conditions under which the vehicle is operated. Furthermore, since several different specifications (variants or versions) of a given model may be grouped together in the list, the figures used in this guide should be treated as indicative only.

A definitive figure for a given specification of vehicle will be available at the point of sale.

Member

As a recently retired ADI, fuel consumption was an important cost element. I found that strangely my first car, a Mazda 323 diesel, was marginally more economical than my second/third cars, both Renault Modus 86 dCi diesels. The Modus has a fuel consumption mode in its on-board computer which generally shows 54mpg + or – 3mpg. Checking this against a calculated full tank to full tank consumption, it was 5 to 10 % optimistic, i.e. real mpg was around 48.
Whether this is better or worse than the manufacturer’s figures does not bother me. It is still a very economical figure.
As a matter of interest I calculated how much the Mazda had saved me over 110,000 miles compared to what a petrol version loaned to me for 2 days would have burned. The answer was of the order of £2,500.

Member
David says:
23 April 2015

Real world Driving Economy, but whose real world driving economy are we talking about.

my wife & I can drive the same car on differnt days and get very differnt results.

This is due to many factors that cannot be replicated in a laboratorys test conditions.

Traffic; heavy left foot, Ambeint Temperature.

I have always used a simple calcuation for the last 20+ years and been within 1-2% accuracy.
Add the Urban + the combined figures and divide by two.

this has worked for me to calculate worst case “real world” economy for many vehicles I have driven over the years before buying them and I have always bettered that MPG with my driving style.

Member

Which? magazine’s promotion for the May edition reads
“False fuel economy
Did you know your car fuel could cost up to £854 more a year than expected because official fuel-economy figures are economical with the truth. Try as we might, it seems even a magician couldn’t produce the same rates stated on manufacturers’ websites. Find out why the system is broken…”

A pity I think that Which? resorts to this kind of publicity. It is well known and acknowledged officially that the EU test cycle does not, and is not claimed to, represent real life economy but to produce comparative data under strictly controlled conditions. It is not “manufacturers data” but EU data they are obliged to use. It also includes emission data which is necessary for cars to be appropriately classified for which standardised conditions are essential. Which? should make this clear instead of rubbishing it just to make publicity. If you want a better idea of what you might achieve in practice use the information published by the good motoring press and Which. But a lot still depends upon factors outside the testers control – your driving style, your journey patterns, the sort of topography where you live for example.

Member

I have yet to read the article, but hopefully my postman will arrive soon.

I acknowledge that EU tests are carried out under standard conditions and most of us are unlikely to achieve similar figures for fuel economy. Thus they are unrepresentative but of value in comparing the fuel economy figures of different cars when deciding which to purchase.

An obvious way of making the EU test results a better indicator of what the motorist could expect to achieve would be to use the same multiplier to produce fuel economy figures that are more realistic. That would mean that some drivers would produce higher and others lower fuel economy than the published figures.