General Motors' claim that the Chevrolet Volt will average at least 230mpg in city driving has not been verified by the US body that measures fuel economy.
Speaking at a news conference yesterday, GM chief executive officer Fritz Henderson explained that the US authorities have reached a draft legislation for how to rate the fuel economy of plug-in electric vehicles.
Using this new calculation, he said the Volt will be rated at 230mpg in town and more than 100mpg on average when it goes on sale in November 2010.
However, details of how the figures are calculated were not made public, and now the Environmental Protection Agency, which sets the fuel consumption tests, has issued a statement to Green Car Advisor regarding Henderson's claims.
"The EPA has not tested a Chevy Volt and therefore cannot confirm the fuel economy values claimed by GM," it said.
"EPA does applaud GM's commitment to designing and building the car of the future - an American-made car that will save families money, significantly reduce our dependence on foreign oil and create good-paying American jobs.
"We're proud to see American companies and American workers leading the world in the clean energy innovations that will shape the 21st century economy."
GM has emphasised that it never claimed the figures were official, but added that it expects its claims to be verified once the EPA tests have been completed.