/ Motoring

Can you rely on your car’s mpg claims?

Empty fuel gauge

Car manufacturers use official miles-per-gallon figures to promote their most efficient models. Yet our recent head-to-head test found that some ‘eco’ cars’ mpg ratings are way off the mark with everyday motoring.

Our research shows that rising fuel prices are one of consumers’ top financial concerns. So making sure you buy a car that doesn’t guzzle petrol or diesel is a higher priority than it ever has been before.

As a result, ultra-efficient small cars have become extremely desirable. The current range of ‘eco’ cars available that claim to emit less than 100g/km of CO2 seem to be the models to choose from, as they also mean free car tax and exemption from the London congestion charge.

Miles off the mark

Worryingly, of all the cars we tested in 2012, the ultra-efficient small cars showed the greatest difference between our own mpg tests and the EU tests used by manufacturers.

In fact, all 10 of the cars with the largest disparity between claimed mpg and our test results are ‘eco’ models that claim to beat the 100g/km CO2 emissions mark. Seven of these 10 claim to achieve more than 70mpg – but our test reveals this isn’t always the case.

The worst is the Peugeot 208 1.4 e-HDi EGC automatic, which our test found to be an astonishing 21.7mpg short of the claimed mpg. It’s still a highly efficient car, but the difference between the EU test of fuel consumption and ours could cost drivers £327 a year in petrol.

However, the biggest cost difference revealed by our tests came from the Fiat Punto TwinAir at £366 a year. The disparity between mpg ratings wasn’t as large, but because the tested mpg was the lowest of all, you’ll have to fill the Fiat more often than the others.

What’s the difference?

One reason for the large disparity between the figures is the difference in our own test cycle compared to the EU cycle that manufacturers use for their official claims.

Like our test, the EU test uses rolling roads. However, unlike EU tests, we use the same rolling road in our lab for all cars to make sure results are directly comparable. We also test any cars with adaptable settings (eco mode, sport mode, town mode) in the default mode the car starts up in – unlike the EU test, which uses the most economical mode.

On top of this, our urban and extra-urban test cycles are conducted from both a cold and warm start and then averaged. It also includes a section of motorway-speed driving. Finally, our mpg figure combines the urban and extra-urban test, together with the motorway test, weighted 70:30. All of these factors make our tests very robust.

Do you know how close you are to actually achieving the official mpg figure for your car? How important is the mpg rating to you when choosing a car?

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

Loading ... Loading ...

Car manufacturers have to declare extra-urban, urban and combined fuel consumptions according to EU standardised tests. These are known not to be representative of “real life” consumption. This is not the manufacturers publishing misleading information, but a deficiency in the EU test conditions – or perhaps better described as a penalty of having to standardise test conditions. At best they can be seen to compare vehicles relative performance to one another.
Therefore tests such as Which? perform, and aggregate data from drivers, give a more realistic picture of what to expect.

No, because in neutral, the car is using fuel to keep the engine ticking over, but in gear with your foot off the accelerator it is using no fuel at all.

I thought that manufactuers’ fuel economy tests are more realistic than they used to be but Rob’s introduction suggests that that might not be the case. Maybe they are only useful to compare the relative fuel economy of different versions of the same car.

It is good that Which? publishes more realistic figures. One of their tests includes a cold start and anyone who bothers to look at the information on their dashboard will know just how thirsty cars are when the engine is cold. There could be a case for including a cold start in every test since that is representative of how most of us are using our vehicles.

It would be useful if MOT testing stations were to collect information about actual fuel consumption for vehicles presented for testing if lifetime information is stored by the computer.

When I was a child my father used to allow his Austin A35 to coast downhill when it was safe to do so, and I have done the same on some familiar roads. I don’t do this on unfamiliar roads or ones where I would have to brake to control the speed. Some may say that I am irresponsible but those who do the same as me will know that coasting can be done safely and can even help achieve the fuel economy figures published by the manufacturer.

I believe coasting out of gear is illegal? I also seem to recall that a diesel engine uses more fuel coasting than it does in gear. It seems counter-intuitive but I think it was to do with the way fuel was delivered. Anyone know?

NukeThemAll says:
16 December 2012

If by ‘coasting’ you mean putting the car into neutral so that the engine is idling freely independent of road speed…..then yes, for modern cars this is less economical. The reason is that the car’s ‘electronic brain’ knows the mismatch between road speed and accelerator position and will simply stop supplying any significant amount of fuel to the engine.

Coasting used to be frowned on for a number of obvious safety reasons, including the lack of engine braking on a descent. However, modern cars are often so highly-geared in top gear, there is negligible engine braking in most circumstances anyway.

Coasting certainly can save fuel on a diesel engined car. The fuel consumption indicator makes this very clear. I don’t know about the legality but I only coast where I know the road and not when I have a passenger in the car. The argument is that you are not in full control of your car. That would certainly be the case if you were foolish enough to stop the engine.

With rising fuel prices, I would not be surprised if manufacturers introduce coasting as a feature of models with automatic or semi-automatic transmissions. There are safety features that could be built in to minimise the risks.

My father also taught me to put my foot on the clutch before starting a car engine, for safety and to disconnect the gearbox and put less load on the starter. I knew I must never do this when there was an instructor or examiner around. When I bought a new car recently the salesman warned me that I would have to remember to put my foot on the clutch when starting the engine. I said that I probably would not have a problem, having done this since 1968.

Interestingly, back in the 60’s when Saab still produced cars with 2 stroke engines, they actually fitted a freewheel mechanism which allowed the car to coast when you took your foot off the throttle. They did this to improve the awful economy associated with a 2 stroke and also to reduce the smoke from the exhaust.
The danger with coasting is the risk of the engine cutting out – rare perhaps, but it can happen, but when it does, you might lose the power assistance to the steering, making it very heavy – and you can lose the vacuum assistance to the brakes – reducing their stopping power.

I have read that other cars would coast too, though I don’t know the details.

Yes the engine could cut out when idling, but I can’t remember ever this happening to me, even when stationary. Maybe that’s why the Highway Code does not even mention this. If it did happen the dashboard warning lights would alert me to the problem. You are right about loss of power assistance to steering (one reason why I only coast on straight roads) but the vacuum reservoir provides power assistance for two or more applications of the brakes. I periodically do check the effectiveness of my brake servo and handbrake in deserted car parks to ensure they are working well.

I am certainly not promoting coasting but I doubt if I am more of a danger than those who allow themselves to run out of fuel on the road. A good habit that I learned from my father is to fill up promptly and so far I’ve not run out of fuel.

I believe it’s an urban myth that coasting saves fuel, and it’s dangerous. Just taking your foot off the accelerator while still in gear does save fuel though, as does a light touch on the accelerator of course.

If you acknowledge that taking your foot off the accelerator while in gear does save fuel, then is it not reasonable to think that you will save more fuel without the drag of the engine slowing down the vehicle?

I’m a cautious driver and far more concerned about the danger of overtaking. I have checked the Highway Code and there are warnings about the dangers of coasting and overtaking.

I wish I had not raised the matter since driving technique is not the subject we are supposed to be discussing.

Iin neutral, the car is using fuel to keep the engine ticking over, but in gear with your foot off the accelerator it is using no fuel at all.

Oh dear. I must just be imagining that I’m getting more miles per gallon when I coast. Let’s drop this discussion – please.

NukeThemAll says:
16 December 2012

Back on the subject of ‘real-life’ fuel economy……the other thing to remember is that it is actually very difficult for the average owner (even if they can be bothered) to get an accurate economy figure.

I know from a comparison with my sat-nav that my vehicle speedo over-reads by ~10%. Making assumptions that the trip computer or even a simple mileometer may be equally in error, it would mean that the trip computer mpg figure is 10% optimistic and doing the sums manually by ‘brimming’ the tank and recording mileage, is equally inaccurate.

A sat-nav, which is of course pretty damn accurate for speed and location, would be the only trustworthy ‘true’ mileometer (or perhaps reading the mileage from a trip planner such as Google Maps – but I’ve never researched the accuracy of their claimed journey distance).

There have been countless posts on the Which? conversations regarding how to drive and maximise fuel economy (but without doing silly things like driving at 30mph on a de-restricted road and annoying everyone) so I won’t refer to or repeat them. However…..I do find that driving with a fair degree of care and mechanical sympathy allows me to get close to or even exceed the manufacturer’s claims for my car.

Given the way that most of the local drivers hurtle about, fuel economy and hence price of fuel are simply not factors in their day-to-day thinking……

Agree with Nuke.. the mpg displayed on the onboard computer is 5-10% higher than the one I get by calculation on tank filling ( over several tanks full) , this is on a Skoda Yeti my old VW Golf mk4 figures didnt seem to show this difference.
The speedo also again does over-read by almost 10% .
However if it was simply a matter of the mileometer being wrong I wouldnt get a discrepancy between the displayed mpg and the calculated one !

Not sure how accurate sat-navs are especially at lower speeds and in town, the gps location can be a few metres out and would result in a more zig-zag path (and longer) than actually is driven. However I suppose with suitable smoothing this error could become negligible

Whatever methods are used to provide “realistic” mpg figures, individuals driving habits and journeys vary so much as to make them pretty academic.

If my comments about coasting are regarded as irresponsible the moderators may wish to remove my messages on this Conversation. I would like to make it clear that I am not suggesting anyone should break the law or do anything that could risk injury to themselves or other people.

It would also be a good idea to get back to the topic of manufacturers’ claims for fuel economy.

Peter C says:
17 December 2012

A comment posted on 16 December speaks of putting your foot on the clutch when starting the engine, and says “I knew I must never do this when there was an instructor or examiner around”. Why not? Personally, I always declutch and put my foot on the brake when starting the engine for the reasons given by the commentor, and this also allows me to know that the brake servo is operating before I move away – and I am an examiner. Good road observation is the best way to improve fuel economy – releasing the accelerator in good time when a traffic light ahead is red, for example. What is the point of burning fuel to get to a red light? There is also the possibility that it will change through to green before I stop, thus saving fuel and giving a smoother ride to passengers into the bargain. “Slow to flow” is the slogan sometimes quoted. There are many occasions when good planning can save fuel.

We are a bit off-topic but the way we drive certainly has a big effect on fuel economy. I do the same at traffic lights but it does not seem to go down well in central London, where some drivers get upset if you start to slow down before they consider it necessary.

I do not understand this article at all. The gallon is not used in the UK, it has not been used in 30 years.
Why is this being discussed? Surely it should be “why are manufacturers still using MPG figures?”. It means absolutely nothing to me.
Miles per litre would be a lot more useful.

As far as I know, all manufacturers publish fuel economy figures in both mpg and litres/100 km in the UK. Many of us are keen on metrication but until miles disappear from road signs, most of us will continue to refer to mpg.

The graphics in Rob’s introduction make the differences clear, irrespective of what units are shown. If you really can’t cope with mpg, just assume that the manufacturers are being a little optimistic in their claims.

My car will display fuel economy figures as litres per km and litres per 100 km.

Neither miles per gallon nor litres/100 km makes any sense in a country that uses miles and litres but not gallons nor km.
As I said, why not miles per litre? That is how I work mine out.
Just how many here know what mpg actually means? How do you relate that to a gallon of petrol?

Which? has kindly provided a litre/gallon converter on their website:

Those of us who can remember fuel prices in shillings and pence might resort to a quaint system called mental arithmetic that was popular in the dark ages.

My car (Golf 1.6 D) was doing about 13.5 miles per litre but with the recent cold weather and short journeys it’s down to a depressing 11.5. 🙁 The ‘combined’ figure given by Which? is 12.9 miles per litre, a bit more realistic than the manufacturer’s figure of 13.8 miles per litre.

The Which? converter uses an imprecise conversion factor of 4.55 instead of 4.546. This is a very small difference, so I’m being pedantic, but the website incorrectly says that 4546 litres is 999.12 gallons when the real figure is 1000.

It’s probably an age thing, but I use mpg and have a feel for what is good and what is bad consumption. I wonder how many people have the same feel for litres/100km? I suspect the only reason we see fuel priced in litres is that it disguises the cost per gallon. The younger generation have a feel for metric measurements because they are educated to use those – this is a bit like losing a grasp of the value of goods when we went from lsd to decimal currency.

I’m sure you are right, Malcolm. Part of the problem is that increased mpg is good whereas in litres/100 km, a lower figure is better. Maybe we should have gone for miles/litre, as suggested by brianac, when fuel sales moved from gallons to litres.

Just what “Age thing” is this Malcolm?
I still rememer petrol going up to 3/6 a gallon when I filled my motorbike, it does not mean I want either gallons nor £sd to return.
The fact is we no longer use either and have not done so for 30 years.
Live with it and get over it.

The point I was making was that people brought up – educated – in a particular era still can relate to the values of that era – as, for example, in evaluating the significance of miles per gallon. Incidentally, as far as I am aware, gallons and miles are still UK measures (or pints, if you drink). I have nothing to “get over” as you put it, as I am quite capable of working in metric or imperial.

OK, thats fair enough. I too was educated for the most part in Imperial. I could not wait to get rid of its shackles and start using something a little more logical. I do not find it so easy to work in both units, I don’t want two incompatible units thrust on me such that I really understand niether. I do not like references to ‘my age group’, and ‘waiting for us to die out’ as being reasons for this country being held back. The gallon was removed from UK allowed primary units of measurment from 30 September 1995. It has been illegal for trade from that time. I see cars as trade, mpg displays are part of that trade. Peace.

Some people do brim to brim fill ups to calculate their MPG, or use a fuel calculating website. I don’t do it myself though, I couldn’t be bothered.

I’d say the MPG is affected by individual driving style, for example I remember there was a hybrid MPG challenge in a local dealership, the participants all drove the same car on the same route but got different MPG results.

Andrew G says:
21 December 2012

So why haven’t Which? lobbied the EU to make the economy tests more representative of real life?

There’s a clear example from the USA where the economy testing regime was changed precisely to make the results match the public’s experience more closely.

I don’t think the EU would listen to Which?

The Fiat Punto TwinAir is a petrol car – not diesel.

Moving ever so slightly away from the original topic but in reply to the ‘MPG v Kilometres per 100 litres’ topic…. I drove a local Rapid Transit system which was the first in the country constructed in metrical units ( Newcastle on Tyne’s Metro) As forward thinking planners had everything one could see and touch metricated, Kms for speed limits, metres for Metro car measurements and metric Tonnes for weights, it often amused me when, after a breakdown in service, the driver had to fill in a Lost Mileage report!

With more and more cars these days having on-board “computers”, it is often possible to set the display to give a constant read-out of miles per gallon. Put your foot down and the figure instantly plummets, but ease off, eg when going down a slope and the figure can soar, even off scale. Whilst not totally accurate, seeing this figure displayed as you drive does have a positive effect on how you drive, without going too slowly or causing a nuisance.

I think the constant fuel consumption readings can be a bit misleading. They do indeed show your instantaneous fuel consumption but it doesn’t necessarily mean that minimizing it at all times will lead to better overall consumption on a complete journey. For example, when going downhill, the reading will show a very good consumption figure indeed – but all this means is that you are using up a different form of energy instead of petrol to maintain your speed. You are using up potential energy, in the form of altitude. Eventually, you will have to go uphill again, and fill up your lost potential energy reserve by using up more petrol (with very low mpg figures). Similarly, putting your right foot down to the floor will show a very poor instantaneous consumption figure but in fact it won’t result in a poor overall consumption because you’ve converted petrol into kinetic energy (speed) so haven’t in reality thrown energy out of the window. Indeed, by accelerating fast, you can afford to keep your maximum speed a little bit lower and still reach your destination at the same time, and by having a lower speed you can save fuel.

Jehosophat says:
21 December 2012

I find if very odd that manufacturer’s MPG and CO2 ratings are for a test cycle that is basically very short, slow trips. I wonder what % of the miles driven is in fact on the motorway? It is really hard to get figures for economy for a steady 70mph for modern vehicles, and yet surely this is how most of the miles for medium to high mileage drivers (and thus most of the miles full stop) will be driven.

I have owned various cars with fairly large normally aspirated petrol engines (2.8, 3.0. 3.2 litres) and they can easily better the quoted EU mpg at motorway speeds. And it is in this sort of driving that I find the small cars with supposedly amazing mpg really suffer – many can barely better 40mpg. I can do that at a constant 70 in my 3-litre automatic petrol estate car (trips up to 42mpg if I drive like a saint – 38mpg EU).

As I do a lot of miles but almost never short trips in my cars, it is really annoying that the quoted MPG figures bear no relation to how I will use a car.

brian.t says:
21 December 2012

I venture to suggest that all new cars are becoming ‘keyless start” now, consequently you have to decluch to start the thing! Regardless of what posters are quoting on here, I know in that 1957 I paid 19/10p for 4 gallons (18.18 litres!). 18 litres and change out of a pound! Those were the days.

Brian, even some cars which aren’t ‘keyless start’ require the clutch to be depressed when starting. I had a 2003 Yaris where this process had to be followed and it wasn’t a keyless entry car.

If you go to honestjohn.co.uk you can get a real world idea of what mpg other drivers get in your car, which the site compares to the manufacturer’s figures. This tells me, for instance, that my car has one of the worst discrepancies between manufacturer’s claims and real world figures. I know I’m not going to get the manufacturer’s figures but such large discrepancies are unacceptable, and indicate to me that the tests are seriously flawed, and/or that some manufacturers know how to “cheat” the tests to look good in their brochures.

I don’t know what has recently changed with the tests but for years my daily journey used to beat the claimed MPG by a few MPG. Lexus reckoned I should get 28mpg but I actually achieved 32. Then comes the Honda Civic where It’s gone the other way and I’m 4 mpg under.

I think this is due to what relative percentage of your journeys are in town, on single-carriageway extra-urban roads (whose speed limits are best for economy) and motorway. Some cars are better suited at higher speeds, others at lower speeds.

From the description of how the EU and Which? test their cars, I don’t understand how a rolling-road test accounts for the fuel needed to counteract aerodynamic drag. Can someone explain?

Clint, it doesn’t. I presume the EU tests should not be regarded as realistic of everyday performance, but are standardised tests to ensure all vehicles are tested on the same basis. Apart from being unrealistic, this also can lead to designers developing vehicles that perform well under the artificial test conditions, but not necessarily in real life. Best to use car reviews.

Malcolm, if the EU and Which tests neglected drag, they would be meaningless, for it would be impossible to compare mpg figures between two different cars, and that is exactly what those tests are designed for. For example, if they just measured engine and drivetrain efficiency then they would show that a square-shaped van had better economy at motorway cruising speed than a streamlined luxury car, when the reverse would be true.

Dave Walker says:
27 December 2012

I would like to raise the issue of tyre choice \ wheel size in the MPG discussion. I am driving my second 2.0 TDI 140 Diesel Ford S Max. The first new car 06 achieved 46mpg if driven carefully. The second has never achieved 40mpg regardless of how carfully you drive it. The only difference between them is wheel size and width. The newer car has 18 inch x 235 x 45 tyres. I am convinced the wider tyres cost MPG and this is not pointed out in any official figures when you buy the car. The second point is that each replacement tyre costs over £250 each!! Combine this cost with the lower MPG and you are looking at thousands of pounds over 4 years motoring. Options in the brouchure should carry with them the true costs and implications.

Whilst a change in tyre size / width / make etc is likely to have a bearing on MPG, it might also be the fact that no two cars / engines are exactly the same in performance / MPG etc.