The Trump administration finalized new fuel economy standards for automobiles on Tuesday, moving forward with one of its most sweeping environmental rollbacks even as it contends with an unprecedented national health crisis.

The regulation, scheduled to take effect later this year, will undo an Obama-era policy requiring automakers to sell cars that spew fewer smog-forming pollutants and less heat-trapping gas, which would have led to more electric vehicles and fewer big gas guzzlers.

The new federal rule calls for a 1.5% annual boost in vehicle fuel efficiency, down from 5% under the current rule. By 2026, new cars would have to average about 40 miles per gallon instead of closer to 50 miles per gallon.

President Trump has long maintained that the nation’s fuel economy standards are too costly. The U.S. Department of Transportation and the Environmental Protection Agency, which announced the new regulation after nearly two years working on it, said that automobile prices would spike if the standards didn’t change and that Americans would be forced to drive older, less safe vehicles.

“This rule reflects the department’s No. 1 priority — safety — by making newer, safer, cleaner vehicles more accessible for Americans who are, on average, driving 12-year-old cars,” Transportation Secretary Elaine Chao said.

Environmental groups countered that any savings at the car dealership would be offset by the need to buy more gasoline with a less efficient vehicle. They also point to the hidden costs of dirty air and atmospheric warming. The transportation sector remains the nation’s biggest driver of climate change.

“This doesn’t benefit anybody except oil companies,” said Maya Golden, deputy director of the Climate Law Institute at the Center for Biological Diversity, an environmental advocacy organization. “It’s going to lead to a huge amount of climate pollution.”

Models that Golden has looked at suggest that more than 1 billion metric tons of carbon emissions will result from the dirtier cars built under the more relaxed standards, the equivalent of operating 68 coal plants for five years, she said.

Golden also said that it was insensitive for Washington to roll back pollution controls as the nation grapples with a deadly respiratory illness spread by the coronavirus.

While combating the coronavirus outbreak remains the stated priority of the White House, the administration is also coming up against the end of its term and a reckoning that major policy changes can take months to secure.

The weakening of the nation’s fuel economy standards is just the latest of dozens of efforts by President Trump to gut climate policies and roll back protections of air, water and wildlands. The new effort, though, is one of the most consequential, with big stakes for California.

California has long set its own mileage standards, getting a waiver from the federal government to operate with tighter rules because of its famously filthy air. The national standards rolled out by the Obama administration in 2012 were designed to conform to California’s rules so automakers wouldn’t face different regulations in different parts of the country. In September, however, the Trump administration revoked California’s waiver, which could leave the state at the mercy of the new federal policy.

California and 22 other states that support stronger fuel efficiency standards are fighting in court for California to keep its waiver. The new national mileage rules are also certain to face a legal challenge as critics seek to hang on to the regulation established under President Obama.

“We hope that everybody will sit down and review our analysis and understand that this is the right balance to strike for the American people,” said James Owens, acting administrator of the National Highway Traffic Safety Administration, part of the Department of Transporation. “So we hope that we will not see litigation, but of course we are prepared to defend the rule if such an unfortunate eventuality should arrive.”

The Trump administration, to win in court, must show that the new standards live up to its claims that consumers will benefit, something that legal observers say could be difficult to do.

“When you revisit an administrative action like this, there’s a heightened level of scrutiny,” said Julia Stein, an environmental law professor at the UCLA School of Law. “The administration has really struggled to provide an adequate basis to roll back the Obama-era standards.”

The legal wrangling also creates uncertainty for the auto industry. With the standards in limbo, manufacturers are unclear how to proceed on their 2022 fleet, which they’re planning now.

Ford, Honda, BMW of North America and Volkswagen Group of America have pushed for tighter standards, even agreeing to comply with California’s stricter policy. Other automakers have sided with the Trump administration, seeing the current regulation as burdensome, though all car companies are in agreement that having two different rules doesn’t work.

“The auto industry has consistently called for year-over-year fuel economy and greenhouse gas improvements that also recognize that the standards originally developed almost a decade ago are no longer appropriate in light of shifting market conditions and consumer preferences,” said John Bozzella, president and CEO of the industry’s largest trade group, Alliance for Automotive Innovation, in a statement.

The new standards are slightly stricter than the outright freeze on fuel efficiency requirements, at 2020 levels, that the Trump administration had initially considered.

If the White House prevails in what’s certain to be a years-long legal battle, the new regulation would mark the biggest blow yet to the nation’s fight against climate change.

“We’re looking at very serious impacts,” Stein said. “We will go from having one of the most aggressive fuel standards in the world to having one that lags behind many other countries, including South Korea and India.”

Chronicle Staff Writer Tal Kopan contributed to this report.

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander