Carmakers are considering challenging in court California’s plan to require vehicles sold in the state to cut greenhouse gas emissions almost 30%, an industry group reportedly said.

Reuters said the draft plan of California, which accounts for nearly 13% of the US vehicle market, would phase in reductions of gases linked to global warming in two steps for cars and trucks sold in the state from 2009 through 2014.

But the carmakers reportedly believe the state’s plan, which comes at a time when US petrol prices are close to record highs, would raise vehicle prices by forcing them to boost fuel economy.

“It is illegal,” Eron Shosteck, spokesman for the Alliance of Automobile Manufacturers, told Reuters, adding: “States cannot set their own fuel economy standards.”

The new proposal is likely to add “thousands of dollars” on each vehicle, Shosteck reportedly said.

Reuters noted that the alliance represents nine carmakers including Detroit’s Big Three, whose profits come mostly from sales of poor-economy sport utility vehicles and pickup trucks.

To cut emissions, carmakers must improve fuel efficiency since greenhouse gases such as carbon dioxide cannot be filtered from vehicle exhausts, the news agency said.

Reuters said that General Motors and DaimlerChrysler have already fought California in the courts to delay clean air regulations – in 2002, they won a court injunction that postponed California’s plans to force carmakers to sell a certain number of low-pollution vehicles.

Last year, GM and DaimlerChrysler dropped their lawsuit after California changed its policy to allow more hybrid vehicles, which use batteries to cut petrol consumption, the news agency said, noting that many carmakers, including Detroit’s Big Three, have plans to roll out hybrid vehicles.

Despite California’s plan, fuel economy is still not on top of consumers minds, however, Shosteck reportedly said, adding: “It still ranks behind cupholders.”