Toyota is scrambling to protect its green reputation in the US, its largest market, where environmental groups are urging it to drop its opposition to a draft fuel economy bill.
The controversy has landed the maker of the world's top-selling hybrid vehicle, the Prius, at the centre of a high-stakes legislative debate, and has even elicited a defence on its behalf from General Motors, its rival for the position of the world's top carmaker.
Nine groups, including the Union of Concerned Scientists, the National Environmental Trust and Friends of the Earth, last week asked Americans to sign a letter urging Toyota to drop its opposition to a draft bill on fuel economy standards passed by the US Senate.
The letter, addressed to Shigeru Hayakawa, president of Toyota USA, claims Toyota is joining its Detroit rivals to deny Americans the strict fuel economy standards it complies with in Japan.
"When I decide what type of vehicle I want to drive, I take a company's overall reputation into consideration," it reads.
"Toyota's credibility is on the line."
Toyota has joined GM, Ford Motor and Chrysler in opposing the fuel economy bill in the Senate.
The draft requires manufacturers to reach a fleet average of 35 mpg by 2020 - 10 mpg higher than today. But the carmakers say the target is unrealistic and would in effect limit production of pick-up trucks and other large vehicles.
Toyota and its US rivals support an alternative bill proposed in the House of Representatives that sets a goal of 32 to 35 mpg by 2022.
"We believe this legislation sets a realistic and achievable deadline for all automakers to increase fuel economy standards," said Josephine Cooper, Toyota's US head of government affairs.
Alongside the Prius, Toyota produces many larger vehicles, including the Tundra large pick-up truck.
Carmakers are wary of bearing the full cost of transforming the way Americans drive, which they say will only change dramatically when petrol taxes are raised.
Thomas Friedman, the New York Times columnist, asked this month: "Why would Toyota, which has used the Prius to brand itself as the greenest car company, pull such a stunt?"
But GM spokesman Tom Wilkinson defended Toyota.
"There's nothing sinister about Toyota - or anyone else - building trucks," he wrote on GM's FYI blog.
"To the unending frustration of Mr Friedman and a handful of environmentalists, Americans buy trucks."
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