/ 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

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Clint, generally, as far as I know, “putting your foot to the floor”, i.e. trying to accelerate hard, wastes fule because your engine is working inefficiently. Best to accelerate relatively gently if you want to improve your fuel consumption.

Malcolm, I think it is a misconception that full throttle wastes fuel. It is probably an over-simplification stated to discourage drivers from doing stupid things like accelerating towards red lights. From the Wikipedia page on Engine Efficiency: “Under part throttle conditions (i.e. when the throttle is less than fully open), the effective compression ratio is less than when the engine is operating at full throttle, due to the simple fact that the incoming fuel-air mixture is being restricted and cannot fill the chamber to full atmospheric pressure. The engine efficiency is less than when the engine is operating at full throttle.”

Clint – it all depends on whether you want maximum efficiency from your car or maximum economy. Using your theory, you could travel all day in 3rd gear at say 60mph to achieve “maximum efficiency”, but you would use much more fuel than doing the same trip at the same speed in top gear (say 5th or 6th).

I think most of us would choose top gear to improve the economy – this is after all the main objective of this subject.

AQ – Efficiency translates directly as economy, as it is defined as energy output as a percentage of energy input. (In this case, output energy is mechanical and input energy is chemical.) But this assumes that everything else is unchanged, so your example doesn’t hold. By saying everything else is unchnanged, I mean that under identical conditions (atmospheric pressure and temperature), and identical engine revs, an engine at half throttle is less efficient than an engine at full throttle. The engine at half throttle will be using a large proportion of its petrol just to keep running (the compressions take away a lot of power) whereas the engine at full throttle will be outputting something useful: acceleration. That’s the theory. In practical terms, it means that economical driving entails accelerating at full throttle to reach your desired final speed in the shortest possible time, and then reducing the throttle to maintain that speed for the rest of your journey in the highest possible gear. (I don’t think we are disagreeing, just interpreting the theory differently. And of course, most people would prioritise safety and smoothness before economy.)

Clint – whilst I am fully conversant with the laws of physics in relation to energy input, output and efficiency, you have drawn a direct correlation to economy with your argument – and all this is theoretical and bears no relation to real life driving, which was the original basis for this debate. Even the manufacturers’ MPG figures cannot be relied upon as to what is genuinely achieved on the road. If you believe you will get the best economy by flooring the throttle in each gear until you reach the speed you want, before easing back, you’re on your own – nobody else subscribes to such a practice for maximising fuel economy and nor would they recommend it. Safety and smoothness should, indeed, be high on everyone’s priorities – and are more likely to be achieved when driving with economy in mind, but if you insist that full throttle acceleration to the desired speed is best – then safety and smoothness, as well as economy have gone out of the window.

Steve says:
1 January 2013

I have a brand new focus ST (not in the slightest a green car, i know!) but the manufacturers MPG claims are way off! I am lucky to get 26 MPG when ford claim over 30 combined.

That’s not too bad. Avoid the Mini Countryman Cooper Diesel – Mini claim 64 mpg combined, but most people struggle to get more than mid-40s.

My brother has a Mercedes diesel. After his previous car was stolen, he replaced it with the exact same model (both were second-hand but with similar ages and mileages.) The second car has noticeably worse economy than the first one.


My vehicle, 3.2 ltr E series M/Benz ’04 saloon is not noted for its frugality. BUT, I think that on a motorway with 69.99mph set and a clear( ish) road ahead, the resulting 46.76 MPG speaks volumes for the auto engineers at M/B This is taken from the car’s computer readings which compare well with the Garmin Sat Nav’s trip stats, and are also borne out on the nifty iPhone ‘Fuel app’, which has returned an average consumption of 39 mpg since May 2012.

Fuel consumption indicators are a great help to demonstrate the effect of driving style but I wonder how accurate they are. Has Which? ever tested their accuracy?

I brim the tank regularly as I don’t get many chances to fill up in the week. Mine is about 3mpg optomistic. I find the instant read out distracts me from the job of driving and it doesn’t really tell me much in the end. When the car goes into its ‘de-clog filter’ cycle the mpg drops for a while and then creeps back up. Its never below 40mpg and never above 45mpg. While on the subject, I find litres per kilometre meaningless since I never use it. If we converted I’d have to make the effort. I expect it would register in time.

I must go back to working out my fuel consumption, as I did in the days before fuel consumption indicators.. How often does a DPF filter go through a cleaning cycle? My car does indicate when this is being done.

About once every thousand miles, but it doesn’t tell me. Surprisingly, where ever I drive and at what ever speed, the MPG doesn’t seem to change. Can’t explain that. ( I got the “it’s” wrong again – sorry. )

Optimistic. Oh bu**er!

Thanks. That could explain why my fuel economy is occasionally very poor.

To get back to the question.
Given there are so many factors that can affect fuel consumption the surprise would be if the manufactuers figures were actually correct.
But it doesn’t really matter because campared to each other you can still tell which car is likely to be the most economical even though none of them will as economic as claimed.
After all you’ll drive in the same maner, usually buy your fuel from the same place and usually take the same routes whichever car you buy. So comparison between the cars you’re thinking of buying is all that counts, and currently optimistic manufacturers figures still do that because they all go about establishing their figures in the same standard way.
The reason manufacturers figures tend to be a bit optimistic isn’t really al that difficult to work out is it?

Well, we all know that we are very unlikely to achieve the economy figures quoted by the car manufacturers, but it does not necessarily follow that if a certain model has a better economy figure than a particular rival, that drivers will also find that the same car still beats its rival in real life. Whilst a car promising 70mpg is very likely going to achieve better real life results than one promising 40mpg, quite frequently another rival promising 60mpg turns out to be more economical than the 70mpg car in real life driving. Although car manufacturers have made significant advances in improving fuel economy, the government prescribed tests result in a grave distortion between the publicised figures and what are really achieved. The figures might be as close as 10% worse on some models, but could be 30-40% worse on others. Interestingly, the greater amounts of difference are usually found on those models that promise the greatest economy, whilst conversely, the claimed fuel consumption of performance cars are usually much closer to what is achieved in real life – and sometimes even exceeded. So the differences probably come down to the manufacturers’ priorities model by model – where the primary aim is fuel economy, you must expect real consumption to be rather worse than the claim. Where fuel economy is lower priority for a particular car, then your own results are likely to be very much closer. As with anything that any government meddles with (of any persuasion) take the figures with a very large pinch of salt – best to rely on real figures.

“the claimed fuel consumption of performance cars are usually much closer to what is achieved in real life” – I’m a bit surprised by that. I would have thought that someone who bought a performance car would be more likely to drive it enthusiastically, with less consideration being given to fuel economy.

The manufacturers consumption figures are produced using standard EU test procedures. Wikipedia says the following:
“In the European Union, passenger vehicles are commonly tested using two drive cycles, and corresponding fuel economies are reported as ‘urban’ and ‘extra-urban’, in litres per 100 km and (in the UK) in miles per imperial gallon.

The urban economy is measured using the test cycle known as ECE-15, first introduced in 1970 by EC Directive 70/220/EWG and finalized by EEC Directive 90/C81/01 in 1999. It simulates a 4,052 m (2.518 mile) urban trip at an average speed of 18.7 km/h (11.6 mph) and at a maximum speed of 50 km/h (31 mph).

The extra-urban driving cycle or EUDC lasts 400 seconds (6 minutes 40 seconds) at an average speed 62.6 km/h (39 mph) and a top speed of 120 km/h (74.6 mph).[21]

EU fuel consumption numbers tend to be considerably lower than corresponding US EPA test results for the same vehicle. For example, the 2011 Honda CR-Z with a six-speed manual transmission is rated 6.1/4.4 l/100 km in Europe and 7.6/6.4 l/100 km in the United States”.

For real life fuel consumption, Honest John (DT) has a website with a useful database.

I don’t believe that these tests involve cold starts, though Which? does include a cold start in one of their tests. Cars are much less economical before the engine reaches the correct operating temperature, so it is hardly surprising that many of us do not achieve the fuel economy figures published for our vehicles.

It might sound surprising Clint, but if you check out what the motoring press find on their road tests and long term tests, their findings do seem to show this. One of the contributing factors is that, very often, the smaller engined versions of cars will deliver worse results than the larger engined versions, if the smaller engined one is driven in a manner to keep up with the larger engined one. Unfortunately, many people make the mistake of trading down to a car that promises more economical motoring, but don’t make sufficient adjustment to their driving style to achieve it. By way of example, the final gearing on cars is getting longer and longer to reduce the revs at cruising speeds (which should reduce consumption). However, an incline, headwind or full load will work against this and more throttle is applied to compensate (almost sub-consciously), when a change to a lower gear would be better. Remember the days when you could pootle along at 20mph or less in top gear ? – well not now, because of the gearing.

Donna says:
29 January 2013

I have a 2012 Astra 2.0 CDTi Sri which I am getting 32 mpg urban and 38 combined. I took it into the garage last week they hooked it up to the computer said there was nothing wrong with it, reset the trip computer then told me they were getting 50mpg by going round the block. I’m a 40 odd year old mother of two! I’m not hammering it and think its a joke. Do I have any comeback as when I bought it they told me 60 + to the gallon combined. Please help x

Donna. It’s all down to driving style. The engine in your car was tested under laboratory conditions before it was ever fitted to your vehicle and produced some very fanciful figures so that the manufacturers could comply with the Law as pertaining to the sale of cars. They are merely a guide and as this conversation has shown, are no where near what would pass for the truth.

Your daily use may be to use the car for several short trips and your style of driving as you typify by admitting your age and status, will be quite gentle. Add the timing of your journeys…(School run both ways in heavy stop / start traffic,) and that will blow large holes in anyone’s attempt to eke out their car’s consumption.

Your local garage mechanic timed his trip ‘around the block’ to enjoy a light traffic time between 9.30 and Midday, plus I’ll bet my best boots that he warmed up the engine first, cleared out the boot and removed the roof rack. ( I obviously can’t know about the last two but if I was wanting to show how well a vehicle performed, they’d be high on the list) Correct tyre pressures, a well serviced engine, clean oil, the list just goes on and on.

Then he’d move out and get into top gear just as soon as traffic conditions allowed, and drive up to the speed limit 50, 60 , 70,mph briskly and time his approach to hazards to use the brakes as little as possible. He’d not be forced to sit stationary for long periods with the engine on, and all in all he’s easily shown you that a higher consumption figure is possible.

I bought a 6 cylinder 3.2 litre car just in time for the current fuel prices to go sky high, but using the methods which I have learned over 47 years of driving in 3 European countries, my car returns a regular 37mpg “round the houses” and a creditable 45 mpg on long trips via motorways.

carl myhill says:
21 February 2013

We have a skoda octavia greenline with a claimed average of 64mpg. When the car was new we could beat this on a long drive (we have kids and rarely go much over 70 and enjoy driving for economy rather than thrills). These days, nearly 3 years later we struggle to get 55mpg on a long run.

My question is, given that we’re all discussing this. What can we do about it? I have bought a product with a claimed performance that it is not meeting. Surely it is not fit for the purpose that it was sold for, right?

Carl, achieving 55mpg on a run with an Octavia isn’t bad going, even if you have had better than this in the past. As discussed earlier on this topic, it is the Government which forces car manufacturers to publicise the results of the three sets of MPG tests, together with CO2 test results – all achieved in strictly controlled “laboratory” type conditions. The tests themselves are specified by the Government. The purpose was to “force” manufactureers to up their game on producing cars which would be more economical and less polluting, so had to be tested in identical conditions, which precludes real roads because of all the variables that would affect the results. The manufacturers have made significant progress in improving economy and reducing CO2 emitions, but have also exploited the potential to tweak various techniques to improve the figures. Apart from increasingly more efficient engines, lowering the suspension a little, fitting “stop/start”, fitting narrower wheels, adding spoilers and other aerodynamic tweaks, electric power steering – are all used in various combinations on different models. However, the changes to gear ratios probably have the biggest influence, after engine improvements. Final gearing is getting taller and taller, producing some spectacular results in the official tests and they will continue to experiment to beat the oppositions’ results. This might look good in their advertising, but bears no relation to real life results. I know a few people who have bought “greenline”, “blue motion”, and other “eco” versions of cars and have found that they are using more fuel than before – quite simply because much of the time they are in too high a gear – fine for steady speeds, but in urban conditions, you have to use lower gears and resist just pushing on the throttle to keep up with the traffic. Few people seem to achieve the manufacturers’ figures and only by driving gently can you get close – no bad thing to try, but difficult to maintain in everyday driving conditions.

As for claiming that the vehicle is not fit for purpose because you cannot match their figures – well you already know how far you would get with that, especially more than 3 years after purchase – nice idea though.

RogerC says:
3 August 2015

My main concern is that my Vauxhall Meriva’s “computer” states that I am getting about 5mpg better results than I am. I always fill my tank each time and note the miles since the last fill making an easy calculation for true mpg. Then reset the “computer”. With around 450miles between fills this gives a fairly consistent figure varying only with some long journeys.

When I hear others say they get high mpg it usually turns out that they are relying on their car’s “computer”.

This debate has veeered all over the world place but I come back to the original point, what use are the fuel consumption figures published by motor manufacturers if they cannot be achieved in the real world? They are deceptions and of limited value for comparative purposes – useless as standalone data.

Surely it would be preferable and more honest for them to publish something that has real meaning, possibly alongside the largely useless EU test data?

I’m thoroughly fed up with the lies, half truths and deceptions with which we are now constantly bombarded – politicians and advertisers being at the top of the muck heap! They all want something from us and don’t care how they go about, truth is the first to be sacrificed on the altar of advantage!

There are big differences between the speed we drive and traffic congestion we experience when driving, hence it’s unlikely that we will ever have official figures that we can rely on, but I agree that we deserve more useful figures.

I predicted that car manufacturers would try and delay the introduction of the WLTP tests and was not surprised to read articles like this one last year: https://www.transportenvironment.org/news/carmakers-pushing-3-year-delay-tougher-co2-tests

Whatever happens, we still need tests that are carried out under standard conditions for all vehicles – and no cheating by over-inflating the tyres and taping up the doors please.

The EC standardised tests – whether the outdated NEDC or the new WLTP – are designed to give you comparative mpgs derived under standard repeatable conditions to give a comparison between vehicles. That is what they state and manufacturers currently have to publish only these official figures (I believe). The newer test is designed to take account of modern car technology and current driving conditions so should be closer to what many people achieve in practice. We all, however, drive differently – how we accelerate and brake, the terrain we are normally in, how loaded the car is, whether we drive mostly town, rural, fast, slow etc etc. The best we can do in addition to standard tests is to accumulate mpgs from as many motorists as possible and present that – either as an average or some other representation. You can find this information on a number of websites, such as Honest John. I’d suggest using that data together with the official data to make an assessment.