Just one in ten cars hit their quoted fuel economy figure while the gap between official lab mpg and real-world fuel consumption is at its widest ever point.

This means that British motorists are unwittingly using almost a quarter more fuel than they would expect, equating to close to 3,000 million extra litres of petrol and 2,609 million litres of diesel every year.

These are the key findings from research by motoring website Honest John, which claims that 13.8 million tonnes of CO2 would have been avoided if cars had met their stated fuel consumption.

BMW’s X5 was revealed to be the biggest fuel economy fibber, achieving just 66.9 per cent of its official mpg, according to figures submitted by consumers through Honest John’s RealMPG website.

Land Rover’s Discovery Sport and Audi’s A4 followed, returning 67.5 and 68.3 per cent of their stated fuel consumption figures respectively.

The top ten list of shortfalling models was completed by Volvo XC90, Fiat 500X, Mini Hatch, Peugeot 308 SW, Range Rover Evoque, Ford Fiesta and Hyundai Tucson.

Just one car surpassed its official mpg: the Mazda MX-5 roadster, which returned 101.5 per cent of its stated fuel consumption.

Next best was Toyota’s Verso and GT86 (99.5 and 98.4 per cent respectively), followed by the Subaru Forester, Peugeot Partner Tepee, Suzuki SX4 S-Cross, Seat Mii, VW Jetta, Suzuki Swift and VW Scirocco.

Honest John’s managing editor Daniel Powell said: “For the majority of drivers, advertised fuel economy figures are quite simply too good to be true. As a result, many are finding it increasingly difficult to understand how much fuel a car will use or how polluting it will be.

“There’s clearly a need for change in the way that mpg is measured, as the real world figures don’t reflect those of laboratories.”

Ben Robb, brand manager at The Fuelcard People, adds: “Shortfalling mpg has been a gripe of motorists and fleet operators for many years now. It’s time something was done.”