It is a gas guzzler reviled by environmentalists but also the only car on sale in Britain that is honest about its fuel efficiency.

Owners of the £90,000 Aston Martin 4.7 litre V8 Vantage may be more interested in the 4.8 seconds it takes to accelerate from 0-62mph or its top speed of 190mph. However, its most unique figure is the advertised miles per gallon.

Every other model among 200 independently tested in realistic conditions failed to meet its official fuel economy rating, with the average car delivering 24 per cent fewer mpg than claimed by the manufacturer. The Aston Martin not only achieved its official figure of 22.5mpg but slightly exceeded it during a “real world" test by Emissions Analytics, which sells data from road tests.

The official test, used by car companies in advertising, is carried out on rolling roads in laboratories, lasts 19 minutes and is conducted mainly at low speed and with gentle acceleration. It has been widely criticised as being too open to manipulation.

Emissions Analytics tests cars on the road for 2½ hours with realistic traffic conditions, speed and acceleration. It found that the gap between the official figures and real mpg had widened since 2012 from 16 per cent to 24 per cent.

Nick Molden, the company's chief executive, said: “While it is true that vehicles went through a phase of significant efficiency improvement in the past, these new results suggest progress in this area has now stalled."

The gap was greatest for small cars with engines of 1 litre or less. They achieved an average of 41.5mpg, 34 per cent less than the advertised average figures. Some of the most misleading mpg figures were those published for plug-in hybrids, the official figures for which optimistically assume that drivers charge their battery fully each day.