Most motorists fail to enjoy as many miles per gallon (mpg) from their new car as they were led to believe.
That is the suggestion being made by Which? following tests on 200 new cars.
And the RAC claims a change to the standard fuel economy test is "long overdue" as it is "clearly not reflective of real world driving".
Just three of the vehicles tested met their official fuel economy level with the average car falling 13% short of its mpg claim, the consumer watchdog says.
And that means, the typical motorist covering 10,000 miles will fork out £133 per year more on fuel than they were expecting to.
Testers found the biggest shortfall on the Mitsubishi Outlander PHEV hybrid. Its official mpg is 148mpg but when they drove it, the model only managed 67 miles for each gallon of fuel.
The Volvo V60 plug-in hybrid and Jeep Grand Cherokee, meanwhile, only managed 80.7mpg and 24.4mpg respectively, compared to their official mpg claims of 155.2 and 37.7.
RAC chief engineer David Bizley said: "A change to the standard fuel economy test used by manufacturers is long overdue as the current procedure is clearly not reflective of real world driving.
"As a result many motorists who have bought vehicles expecting high miles-per-gallon figures have been sorely disappointed when they fail to achieve the numbers claimed by the manufacturer.
"This is not good for either the motorist or the manufacturer and therefore needs to be rectified as soon as possible."
Which? says although official mpg testing is set to be changed in 2017 to make it more realistic, the motor industry is calling for that to be put off until the end of the decade.
It claims the current official testing is outdated and has loopholes that means it does not cover real-life situations such as motorway driving or using air-conditioning.
Richard Lloyd, executive director at Which?, says with fuel costs being such a big concern for motorists the new test should be introduced as soon as possible to prevent drivers being misled by "fantasy" figures.
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, 2% 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.
Copyright Press Association 2015