/ 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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Comments
Member

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.

Member

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.”

Member

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.

Member

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.)

Member

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.

Member
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.

Member

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.

Member

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.