Engines are more efficient than ever, but in most cases our cars still cost us much more to run than the manufacturers’ official mpg figures suggest.
Our independent fuel-economy testing of petrol-engined cars shows that some miss their claimed mpg figure for the combined cycle by more than 30% – with the BMW 4 Series Gran Coupé topping our chart at 33%. That could mean spending hundreds of pounds more on petrol each year if you assumed the official figures were indicative of the mpg you’d achieve.
Below, we’ve listed the 10 cars that stray furthest from their fuel-economy claims, including popular models from Alfa Romeo, Peugeot, Hyundai and Citroën.
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Why the mpg gap?
EU law requires carmakers to show official test figures in their adverts to help consumers compare fuel economy between different models. But we think the figures should also reflect real-world driving, and if a brand quotes an mpg figure in its adverts, a consumer should be able to achieve it in normal use – or at least get close to it.
That’s why it’s vital the EU test is updated to be more accurate in 2017 as planned, and not delayed until 2020 as some manufacturers would prefer.
Petrol engine cars – furthest from claimed mpg
In 2013/14 we assessed 200 cars in our lab and road tests, putting all models through the same fuel economy evaluation. Here we list the petrol engine cars we’ve tested that miss their claimed combined cycle mpg figure by the most.
How we test mpg
The testing that underpins every Which? car review is designed to give consumers the most accurate picture possible of how their car will perform in everyday life. So when assessing fuel economy, unlike the official EU test, we include a motorway driving simulation, switch on the lights and air-con, and inflate tyres to the recommended pressures.
And if a car has different driving modes available, we use the one it starts up in by default, rather than switching to an ‘eco’ mode. This may offer better economy, but will also often neuter a car’s performance to the point where it’s awful to drive.
What’s the problem with EU mpg measurements?
Don’t be fooled by the name of the official driving simulation currently used to calculate mpg figures – the New European Driving Cycle (NEDC) hasn’t been new since it was last updated in 1997, and the basis of the test was first introduced in the 1970s.
The test’s lack of real-world driving scenarios and numerous loopholes make the figures it generates unachievable when you get behind the wheel of a car.