A group of 23 U.S. states led by California, the District of Columbia and some major cities are challenging a Trump administration decision to weaken Obama administration fuel efficiency standards.
In March, the Trump administration issued final rules requiring 1.5% annual increases in efficiency through 2026 - far weaker than the 5% increases in the discarded Obama-era rules - but abandoned its August 2018 proposal to freeze requirements at 2020 levels through 2026.
Last week, a trade group representing General Motors, Fiat Chrysler Automobiles, Toyota Motor and others sided with the Trump administration on its plan and opposed a legal challenge to further weaken the requirements. Other automakers including Ford Motor are not backing the administration plan.
California Attorney General Xavier Becerra said the revised requirements "will increase costs to consumers and allow the emission of dangerous pollutants that directly threaten the health of our communities."
Separately, 12 environmental groups also said they were filing suit Wednesday challenging the freeze, including the Center for Biological Diversity, Environmental Defense Fund, Sierra Club and Union of Concerned Scientists.
The Trump requirements mean the U.S. vehicle fleet will average 40.4 miles per gallon rather than 46.7 mpg under the Obama rules finalized in 2012.
Michigan Attorney General Dana Nessel called the rule a "gift to the fossil fuel industry" that would harm the state, home to Detroit's Big Three automakers, because it would reduce automotive-related employment by 4%.
The Trump administration said the new rules would reduce the future price of new vehicles and save automakers tens of billions of dollars in regulatory costs. It also said they will boost average consumer fuel costs by more than $1,000 per vehicle over the life of their vehicles while lifting oil consumption by about 2 billion barrels and boosting carbon dioxide emissions by 867 to 923 million metric tons.
Last year, the same group of states sued to challenge the administration's earlier decision to revoke California's authority to set stiff vehicle tailpipe emissions rules and require a rising number of zero-emission vehicles. That challenge is pending.
Obama's environmental policies sought to cut carbon emissions that drive climate change, while Trump has ditched numerous environmental regulations his administration deemed harmful to industry.