The Environmental Protection Agency and the National Highway Traffic Safety Administration have unveiled their new proposal for fuel economy standards following the scrapping of the Obama-era plan which called for a corporate average fuel economy rating of 54.5 mpg (65.4 mpg United Kingdom / 4.3L / 100km) by 2025.
The prospect of an extended legal fight has discomfited automakers, who had asked the administration to relax the Obama-era rules but don't want to see the US market split in two, with different models of cars required in blue and red states.
The administration also filed notice Thursday that it wants to revoke the authority of California and other states to set their own, stricter mileage standards - independent of federal ones.
Becerra said he was already preparing to sue the Trump administration and would be joined by 18 other state attorneys general, including Pennsylvania, Massachusetts, and NY.
The state coalition, led by California Attorney General Xavier Becerra, includes legal representatives from jurisdictions that have adopted California's more stringent vehicle emissions standards.
"For more than a decade, ME and the other states have used our rights under the Clean Air Act to limit tailpipe pollution beyond federal minimum requirements", Emmie Theberge, federal project director at NRCM, said in a statement.
Pollution from cars, trucks and other on-road vehicles is the California's single-largest source of greenhouse gas emissions, according to state data. All have Democratic attorneys general.
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Along with states, the auto industry and the EPA's own advisors, the Trump administration is also attracting hefty criticism from environmental groups.
"The fleet of new vehicles today is the most fuel efficient ever, and they have gotten safer every year", said Luke Tonachel, director of clean vehicles and fuels at the Natural Resources Defense Council.
Obama sought to add to the fuel standards in an effort to reduce carbon dioxide emissions and cut down on greenhouse gases, but automakers fought the changes. "It's a boon for big oil that ordinary Americans will pay for with their health and their wallets". The administration also believes the change will shave about 7 or 8 percent off the cost of a new vehicle in the coming years, a savings of around $2,300. Republican Arnold Schwarzenegger pointed out that it was Ronald Reagan who sought out California's right to regulate emissions. "With today's release of the administration's proposals, it's time for substantive negotiations to begin", Gloria Bergquist, a spokeswoman for the Alliance of Automobile Manufacturers, said in a statement to the New York Times.
Environmental and cost impacts of freezing the rules are said to be minimal.
President Donald Trump said in March previous year that his administration is working on relaxing the Corporate Average Fuel Economy (CAFE) standards set by his predecessor.