German car giants Mercedes have been left red-faced again as it emerged the average gap between real-world fuel consumption and lab results for their cars is 54 percent.

The Mercedes A and E classes are burning 56 percent more fuel on the road that is claimed in their sales brochures, according to a report by the Transport and Environment's Mind the Gap, after Emission Analytics found their diesel engines emitted four times the legal limit for nitrogen oxide last year.

Figures show industry-wide statistics are also still skewed with the average gap for cars in Europe at 42 percent, costing drivers around £460 extra a year at the pump.

Mercedes is the worst-performing manufacturer, with the C class burning 54 percent more fuel than advertised, compared to the VW Passat, which is measured at 46 percent. The fuel efficiency tests are conducted in the laboratory with many loopholes and flexibilities and are prone to extensive manipulation.

Greg Archer, clean vehicles director of Transport and Environment, said: 'Cars that burn 50% more fuel than advertised are deceiving consumers and cheating environmental rules.

The average gap across all brands in Europe is quickly growing, jumping from 28 percent in 2012 to 42 percent last year. The gap was 14 percent a decade ago. Audi is the second worst brand with an average gap of 49 percent. In general, most car brands have an average gap higher than 40 percent with Peugeot at 45 percent, Toyota at 43 percent and Volkswagen at 40 percent.
The exception is Fiat, which is still operating at 35 percent higher than advertised.

Distorted test results deceive European drivers whose cars consume much more fuel than is promised in marketing materials.