While the new standard is significantly lower than Obama's 2025 target of 54 miles per gallon, the difference is expected to have a minimal impact on climate.
Transport has become the largest sector source of greenhouse gas emissions in the United States, as cheap fuel has encouraged Americans to buy larger vehicles such as SUVs.
General Motors said in a statement that it wants to work "with all parties to achieve one national 50-state program", adding it was committed to "continually improving fuel economy and our commitment to an all-electric future". Why all of the sudden would the Trump administration - without the facts and the science behind them - feel like they can just yank those away?
While many carmakers have touted their development of more efficient cars, including electric vehicles, auto lobbyists were quick to get assurances from the Trump administration that the more stringent rules would be dismantled.
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 USA market split in two, with different models of cars required in blue and red states.
The changes are considered massive regulatory rollbacks of Obama administration policies that argued requiring more fuel efficient vehicles would improve public health, combat climate change and save consumers money without compromising safety.
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"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.
"My job as the state's attorney general is to protect my state's rights and interests and the environmental rights of all Pennsylvanians", Shapiro said. California and the automakers agreed to the rules in 2012, setting a single national fuel economy standard.
President Donald Trump listens as Rep. Marsha Blackburn, R-Tenn., speaks at a rally in Nashville, Tenn. on May 29, 2018.
The U.S. Energy Information Administration says the transportation sector is the nation's largest source of carbon emissions, and cars make up 60 percent of that sector.
And so what the Trump this is also doing, along with freezing the CAFE standards, is limiting the extent to which states can impose regulations on other states and on manufacturers.
"Unless the Obama administration's punishing standards are changed, consumer choice will be limited and the cost of vehicles will skyrocket", said Republican Senator John Barrasso. As an alternative, the Trump administration proposes holding the standards at those set to take effect in 2020. The affordability argument also ignores thousands of dollars of saving in fuel costs for each driver over the life of a vehicle, opponents of the rollbacks said.
A drawn-out legal battle over the standards could hurt the auto industry as it tries to plan for coming model years.
In 2012, when the standards were first adopted, cars were about 50 per cent of new-vehicle sales.