/ 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

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.

Member

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.

Member

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.

Member

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?

Member
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.

Member

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.

Member

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.

Member

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.