Car fuel economy claims are miles from the truth

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.

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