/ 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

Loading ... Loading ...
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.


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.


“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).”


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.

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.


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.

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..


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:
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.