California and other states which follow its standard have indicated they will take legal action, which could significantly delay matters.
Opponents are concerned about the rollback's effects on air quality.
The states that have adopted California's emission rules together make up about one third of the USA auto market.
The state has struggled to rein in vehicle pollution. But as consumers spend more on gas, the costs would start to balloon.
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The rollback would undermine efforts by California and several other states to meet commitments the USA made in the Paris agreement on climate change.
In their view, the proposed changes will increase daily fuel consumption rise by two to three per cent, but have "no noticeable impact" on smog-forming or other toxic air pollutants.
They noted "it's time for substantive negotiations to begin", and urged "California and the federal government to find a common sense solution". The timeline could then align with the finalization of the fuel efficiency changes, as the proposed rule still needs to work its way through a lengthy public comment process.
The Department of Transportation and the Environmental Protection Agency issued a Notice of Proposed Rulemaking Thursday that officially calls for leaving fuel economy standards alone rather than increasing them as decided under the Obama Administration.
"The law requires fuel economy standards in order to save fuel, and yet this proposal does just the exact opposite", said Shannon Baker-Branstetter, senior policy counsel for Consumers Union, the advocacy arm of Consumer Reports.
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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. The administration's assertion that lighter, more fuel efficient cars are more unsafe has been disputed by transport experts. It might even cause auto prices to stop increasing so rapidly. The firm projects the policy would cost the USA economy $457 billion and cause 13,000 deaths by 2050, as air quality suffers.
The proposal freezes fuel efficiency requirements for America's cars and trucks until 2026 - repealing one of Washington's most significant steps in recent years to curb greenhouse gas.
What were the Obama administration guidelines?The Trump administration's proposal would cut off the average CAFE increases in 2020, when automakers will have to produce cars that get an average of 43.7 miles per gallon.
The rollback has provoked outcry from environmental and health groups, as well as states who are pushing for cleaner vehicle fleets. "They have failed before". "Not only does fuel savings cover the cost of fuel efficient technology, but it also covers all of the other costs that go into automakers annual price increases, including new safety features". "But if not, I'd remind them that California has won this battle before".
The proposal would also end a federal waiver to California that allowed the it to enact stricter tailpipe standards, thereby limiting the types of cars that can be sold in the nation's most populous state.
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Cal Fire Mendocino said on Sunday morning that the Mendocino Complex Fire is at 10 percent containment. "I'm very upset about it". The state spent more than $114 million fighting fires in July, the first month of California's fiscal year.
White House spokeswoman Sarah Sanders said much of the criticism was based on "false" news accounts of the proposal. And when they fail they are prohibitively expensive to fix and often require someone akin to a rocket scientist to troubleshoot.