GASOLINE misers such as the Toyota Prius would get an 'A-' while muscle cars - including the Ferrari 612 - would get a 'D' under a labelling programme proposed by the Obama administration, which wants to convince consumers to buy vehicles that use less energy.

The US Environmental Protection Agency (EPA) and the Department of Transportation (DOT) proposed on Monday that fuel economy labels on the windows of new cars in showrooms compare both mileage and emissions of gases blamed for global warming.

They said the new labels, which they hope will start with the 2012 model year, would give consumers more information about the monetary - and environmental - costs of running their vehicles.

'New fuel economy labels will keep pace with the new generation of fuel-efficient cars and trucks rolling off the line, and provide simple, straightforward updates to inform consumers about their choices in a rapidly changing market,' said EPA Administrator Lisa Jackson.

The proposal requiring two labels would be a big change for consumers and the car industry from the current single energy-efficiency label required on all new cars and light trucks.

One label would measure fuel economy and, for the first time, tailpipe greenhouse gas emissions, and combine those to come up with a letter grade ranging from the 'A+', the most efficient, to 'D', the least, reflecting the grading system common in American schools. No vehicle would get a failing grade.

Gina McCarthy, an EPA assistant administrator, said all-electric vehicles would get the top grade while plug-in hybrid cars, which are charged with an electric power cord and have small engines, would get an 'A'.

The top-performing traditional hybrid cars such as the Prius, made by Toyota and the Fusion hybrid, made by Ford would get an 'A-'.

Ms McCarthy said the EPA was in talks with carmakers on figuring out a miles-per-gallon equivalent fuel-efficiency label for all-electric and mostly electric cars.

The second label on cars in the showroom would include miles per gallon for both city and highway driving, an annual fuel cost for driving the car, and how it compares among all types of vehicles.

The labels will provide consumers with an estimate of the expected fuel cost savings over five years compared with an average gasoline-powered vehicle of the same model year.

In April, the EPA finalised first national limits on greenhouse gas pollution from cars and light trucks and the DOT strengthened fuel economy standards for model year 2012 to 2016 passenger vehicles.

The efficiency rules required that cars and light trucks get on average 15km per litre by 2016, up 42 per cent from current rules.

US passenger vehicles discharge about 20 per cent of the country's heat- trapping gases and consume about 44 per cent of its oil.

One environmentalist said the new labels would help consumers decide which cars are right for them.

'Truth in labelling empowers Americans to make informed real world choices that save our families money, reduce our dependence on oil, and cut heat- trapping greenhouse gases,' said Vickie Patton, the general counsel for the Environmental Defense Fund.

On the other hand the car industry had mixed feelings about the proposed labels. Analysts said car companies had been forced to accept new fuel-efficiency standards released in April after the government bailed them out from financial troubles.

Dave McCurdy, the president and CEO of the Auto Alliance, praised the idea of giving consumers more information, but said 'the proposed letter grade falls short because it is imbued with school-yard memories of passing and failing'.

Public comments will help determine the outcome of the programme. The EPA and DOT are providing a 60-day public comment period after they publish their proposal in the Federal Register later this week.