CARS are falling miles short of their official fuel-economy figures, according to Which?

All but three of the 200 new cars tested by Which? failed to reach their official miles-per-gallon (mpg) figure, with the shortfall averaging 13 per cent.

This meant that on average drivers are spending £133 a year more on fuel than they thought they would.

The vehicle that missed the target by the biggest margin was the Mitsubishi Outlander PHEV hybrid, which is claimed to do 148mpg but only managed 67mpg in the Which? test.

The Volvo V60 plug-in hybrid, which was supposed to do 155.2mpg, came up well short at 80.7mpg.

The third worst was the Jeep Grand Cherokee, which officially does 37.7mpg but could only manage 24.4mpg.

Official mpg testing, believed by Which? and others to be unrealistic and not representative of "real life" conditions, is due to be changed in 2017.

Which? said the EC was facing heavy pressure from the car industry to delay the change until 2020.

It added: "The official test used by car makers is outdated and contains a number of loopholes that lead to unrealistic figures.

"Manufacturers are allowed to reduce results by four per cent at the end of the test, can opt to only test in a car's 'eco' mode, turn off lights and air-con, and increase tyre pressures above the recommended levels to reduce rolling resistance. It also doesn't accurately reflect real-life scenarios such as motorway driving."

Which? executive director Richard Lloyd said: "The cost of fuel is one of the biggest concerns for consumers which is why fuel efficiency has become an important selling point for new cars. The new test should be brought in without delay so consumers are no longer misled by fantasy mpg figures."

The tests were conducted across 2013 and 2014. The £133 average extra fuel cost was based on driving 10,000 miles a year.

The three models which did match or surpass their official figures were:

:: The Skoda Yeti 2-litre diesel automatic, which recorded a figure of 45.6mpg in the test, two per cent less than the official figure of 44.8mpg.

:: And the Mazda 3 Fastback 2-litre petrol manual and Skoda Roomster 1.2-litre petrol manual, which both matched their claimed mpg figures.