Fleets tests in America of hybrid cars that have been fitted with aftermarket plug-in kits have found the cars are coming nowhere near the 100mpg claims, according to Wired.com.

Seattle city said the 14 cars, which have so far covered 17,000 miles, it is testing are averaging 51mpg.

Idaho National Laboratory is seeing similar results with 104 plug-in hybrids it is monitoring.

The cars have covered more than 300,000 miles and returned an average of between 46 to 51mpg.