Official government fuel figures overstate just how many miles cars can do to the gallon, it has been claimed.
Tests carried out on more than 60 cars showed every vehicle had a lower miles per gallon (MPG) figure than that given in Government statistics.
What Car? magazine, which carried out the research in association with Tesco's filling station, says Government figures showed the Kia Picanto 1.0 2, for example, did 67.3 MPG while the magazine's own test gave a figure of 41.2.
Similarly, the Government figure for the Nissan Micra 1.2 DIG-S Shiro was 65.7 and the What Car? figure was 44.1.
The Government's figure for the Ford Focus 1.6TDci 115 Zetec was 67.3 compared with What Car's 43.1.
What Car? has now launched what it calls a "true MPG" initiative whereby car buyers can log on to a website to check miles-per-gallon figures.
The magazine's editor-in-chief Chas Hallett says: "With rising fuel prices, the MPG issue is high on every motorist's agenda. Countless car buyers are frustrated that they don't match the official government fuel figures.
"True MPG will allow motorists to select a car personalised to their driving needs and budget. We hope that it will become invaluable in the complex and emotional car-buying process."
The magazine tested the vehicles using a portable emissions-measurement system when the engines had reached normal operating temperature.
The vehicles were then driven by two experienced engineers over a variety of roads, including motorways and A and B roads, and through towns and villages. The data from the testing considered driving style, changes in altitude, temperature, humidity and engine temperature.
Local Transport Minister Norman Baker says: "I do not accept the Government overstates the fuel consumption of cars.
"The official test provides an indication of the relative fuel economy of different cars, but is not the whole picture, as no single test can represent the fuel consumption individual drivers will experience on the road."