The true cost of misleading mpg figuresPopular cars could be costing £100s extra in fuel

15 July 2015

Cars from the likes of Vauxhall, BMW and Jeep could be costing their owners up to £850 extra in fuel each year, based on our fuel-economy testing. Read on to find out if you're affected.

We all have our own tactics when it comes to getting more miles from a full tank – whether it’s cruising at a constant 70mph on the motorway, or changing gear efficiently.

But, no matter how carefully you drive, some cars will cost you much more than you may have anticipated, especially if you took the official fuel-economy figures at face value.

Our testing has found models such as the Vauxhall Corsa, BMW X4, Mitsubishi Outlander and Lexus GS could cost you upwards of £350 more each year for every 10,000 miles you drive - enough to pay for the car’s annual service with change to spare.

The worst offender in our study, the Jeep Grand Cherokee, could cost you a huge £854 more to run than you'd expect based on the official mpg claims. That’s the equivalent of a return flight to Florida this winter.

Come clean on fuel claims - sign our petition for fuel claims you can trust.

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.

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.

Does your car deliver the fuel economy you expect? Scroll down for our list of the most misleading models and the extra costs owners could incur.

Difference in fuel costs per year

@inlinemarker@ Difference in fuel costs @inlinemarker@

Annual fuel costs are based on driving 10,000 miles a year and the average cost of fuel for the months March 2014 to February 2015 according to the AA fuel price report (petrol: 125.5p per litre; diesel: 130.0p per litre).

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 makes the figures it generates unachievable when you get behind the wheel of a car.

More on this…