Virtually all cars are failing to meet their manufacturers' claims about miles per gallon, new research shows.
As a result, drivers are spending an average of £133 more a year on fuel than they are being led to expect.
Consumer watchdog Which? tested 200 new cars across 2013 and 2014, and found that all but three fell short of their official mpg figures - by 13% on average. It's calling for more realistic tests to be introduced.
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," says Which? executive director Richard Lloyd. "The new test should be brought in without delay so consumers are no longer misled by fantasy mpg figures."
Research from the Department for Transport & the Environment compared results from the official test used by car manufacturers with those from real life scenarios and found an average discrepancy of 23%.
The reason is that the official test contains a number of loopholes that can lead to unrealistic figures. Manufacturers are allowed to reduce results by 4% at the end of the test and need only test in a car's 'eco' mode.
They are also allowed to turn off the car's lights and air-con, and increase tyre pressures above the recommended levels to reduce rolling resistance. And, on top of this, the tests don't accurately reflect real-life scenarios such as motorway driving.
The car that performed worst compared to its official mpg figure was the Mitsubishi Outlander PHEV, a plug-in hybrid, which overstated its mpg by 120% - costing the average driver £459 a year in extra fuel.
However, Jeep Grand Cherokee owners are being hit even harder, shelling out up to £854 a year more on fuel than they are led to expect.
Other cars that will cost owners a lot more than expected are the BMW 4 Series Gran Coupé (£421), BMW X4 (£419) and Volvo V60 Plug-In Hybrid (£352).
The European Commission has plans to introduce a more accurate test in 2017 - but is facing pressure from the car industry to delay the change until 2020.
"Until a more realistic test is introduced, drivers can check the Which?-tested mpg figures for hundreds of cars in our online car reviews," says Which.