A DECISION by the US Supreme Court could signal the beginning of the end of America’s love affair with big cars, pickup trucks and gas-guzzling 4x4s.

The court has ruled that the government’s Environmental Protection Agency (EPA) can apply regulations limiting the output of carbon dioxide - identified as a greenhouse gas that contributes to global warming. This would bring America into line with the EU, which is considering how to implement legislation requiring the average carbon dioxide produced by new cars to be below 130g/km.

Car manufacturers selling in America have to meet Corporate Average Fuel Economy (Cafe) standards, which are currently equivalent to 33 miles per imperial gallon for cars and 26mpg for pickup trucks and 4x4s.

Environmental lobby groups such as the Sierra Club have advocated revising the Cafe figures to 54 and 41mpg - but the American government, under pressure from domestic carmakers, has rejected such a drastic increase. Sales of vehicles with ever-larger and more powerful engines have continued to thrive while the manufacturers stay within the generous Cafe allowance.

An EPA ruling that restricts the output of carbon dioxide or requires the trading of carbon credits between the makers of economical small cars and the gas guzzlers could curb the industry’s excesses. Hefty macho vehicles like the Hummer H2 with a 6-litre V8 engine, the rappers’ favourite Cadillac Escalade SUV (6.2 litres), and the 185mph Chevrolet Corvette sports car would not be viable if carbon-dioxide limits were severe.

Until now, the American government’s efforts to reduce vehicle fuel consumption have been less to do with the environment and more with a desire to reduce the country’s dependence on foreign oil. With petrol at about $3 per US gallon (40p a litre), good fuel economy is not high on the agenda for most car buyers. Carbon-dioxide figures, which determine the rate of road tax and company car benefit in Britain, are neither widely discussed nor generally understood.

But the state of California has already set limits as part of its pledge to reduce greenhouse-gas emissions by 30% by 2016 and 10 other states have adopted the same standards. It is thought that the House of Representatives’ energy and commerce committee will come up with a bill of climate-change control measures before the end of this year.

The news that the EPA could set nationwide carbon-dioxide limits comes at a bad time for the big three American carmakers — Chrysler, Ford and General Motors - which are all in financial trouble and not ready with rivals for cars like the Toyota Prius petrol-electric hybrid.

The big three are against regulations based on the carbon-dioxide emissions of individual vehicles. General Motors chairman Rick Wagoner said: “We have had Cafe for 30 years and it doesn’t work.” He calls for a different strategy to control carbon dioxide, adopting a higher proportion of renewable fuels, such as bio-ethanol made in America from corn.

GM has put a new range of big saloon cars on hold, awaiting developments from the Supreme Court’s EPA ruling. And just as GM’s thirsty V8 engines are under threat, Toyota, its arch-rival as the world’s No 1 carmaker, has presented a hybrid version of the biggest Lexus saloon. The Lexus LS600h has a 5-litre V8 petrol engine combined with an electric motor and averages 30mpg and 219g/km carbon dioxide - figures similar to the downsized cars and trucks with small four-cylinder engines that American environmentalists are keen to encourage.