ONLY two cars actually managed to pass the advertised fuel economy test, with the average model performing 30 per cent worse than it says on the tin.
The Aston Martin Vantage and 3.7l Nissan 370Z were the only models to fare better on the road than in the lab.
Some of the worst offenders were the most common, including the Fiat 500 and Ford Fiesta which only managed around half of the mileage promised.
The Fiat 500 took the biscuit by rating itself at 70.6 miles per gallon (mpg) but only delivering 39 mpg.
Despite still being more than a fifth lower than its advertised potential, the Honda Civic still clocked up the best on-the-road mileage at 61.8 mpg, while the BMW X5 fared the worst at just 16.2 mpg.
A new testing system is due to come into force later this month, but is unlikely to dent the average 29 per cent gap between advertised and actual mpg, with labs still unable to replicate the meandering hills, tight corners, traffic jams and air conditioning of the real world.