The Environmental Protection Agency (EPA) is considering altering the way it determines mileage figures for hybrid vehicles, which could ultimately result in much higher mileage figures being awarded to hybrids. Currently, the EPA test cycle involves city and highway driving, as well as high speed runs and air conditioning loads. The Chevrolet Volt hybrid currently achieves figures higher than 100mpg (2.35L/100km) on these tests because of its 40-mile electric-only range and the fact that its internal combustion engine is only used around 15% of the time for the EPA test.

But now the agency is considering changing the requirements of the test drastically - the new proposal would require hybrid cars that are tested to be kept at close to full charge, meaning that effectively their electric-only range is eliminated.

To prevent this from affecting the Volt, GM is asking the EPA to classify the car as an electric vehicle for regulatory purposes. This is not entirely unreasonable as the car has already been given preliminary certification as an electric vehicle by the California Air Resources Board (CARB), according to GM spokesman Rob Peterson.

Speaking with Automotive News, Peterson explained that a government rating of more than 100mpg would give GM invaluable marketing boost but also help the company meet upcoming CAFE regulations.

For electric vehicles, the EPA uses a Department of Energy mathematical formula to translate energy use into some equivalent of miles per gallon of gasoline. For the all-electric Tesla, for example, the EPA rating is 244mpg (0.96L/100km) even though it uses no fuel whatsoever. Since the Volt will primarily be driven on electric power alone, as most owners are expected to use the car less than the 40 mile electric-only range, GM is hoping to have the car rated using this formula.

Opponents say that since some owners will be using the petrol engine to generate electric power to drive the motor, the car should be tested using the regular city and highway driving rating. GM is yet to reveal exact mileage figures for the car when using the petrol engine but says it will fall somewhere between 35 and 50mpg (6.7L/100km and 4.7L/100km).