Official government fuel figures overstate just how many miles cars can do to the gallon, according to What Car? magazine.
Tests carried out on more than 60 cars by What Car? in association with Tesco's filling station showed every vehicle had a lower miles per gallon (mpg) figure than that given in the Government statistics.
What Car? said Government figures showed the Kia Picanto 1.0 2, for example, did 67.3mpg while the magazine's own test gave a figure of 41.2 mpg.
Similarly, the Government mpg figure for the Nissan Micra 1.2 DIG-S Shiro was 65.7 and the What Car? figure was 44.1mpg.
The Government's figure for the Ford Focus 1.6TDci 115 Zetec was 67.3mpg compared with What Car's 43.1mpg.
The magazine said it 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, ambient temperature and humidity and engine temperature.
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 said: "With rising fuel prices, the miles-per-gallon 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."