Traffic on U.S. Highway 101 in Los Angeles on Feb. 21. (Justin Sullivan/Getty Images)

IN THE start of what promises to be a broad rollback of federal environmental rules, President Trump announced on March 15 that he has reopened review of an auto efficiency regulation that would have forced the nation’s vehicle fleet to use less fuel and cut pollution and which the Environmental Protection Agency finalized in the closing days of the Obama administration. Promising a Detroit crowd that he would “protect and defend your jobs, your factories,” the president failed to mention the rule’s benefits: curbing the country’s gasoline addiction would shrink fuel bills and reduce the country’s carbon footprint. Nor did he mention the EPA and the National Highway Traffic Safety Administration’s exhaustive studies finding that the rule is technically feasible and that its benefits, particularly in gas savings, would far outweigh its costs.

The auto industry pushed the Trump administration hard to revisit the rule. The nation as a whole will pay the price.

The Obama administration’s fuel-efficiency regulations will stay in place through 2022. But the standards in question, for model years 2022 through 2025, are much more ambitious. Set in 2012 and formally reviewed just before Mr. Trump’s inauguration, the industry’s efficiency target is a fleetwide 54.4 mpg. “Automakers have a wide range of technology pathways available to meet the MY2022-2025 standards, at slightly lower per-vehicle costs than previously predicted,” the EPA concluded as it reaffirmed the rule in January.

In response to the standards already in force, national vehicle fuel economy has begun to rise. In its 2017 Annual Energy Outlook, the Energy Information Administration projected that the government’s efficiency drive would cut gasoline consumption to the lowest level in decades. Overall, without any further tightening of the rules after 2025, transportation-sector energy use and greenhouse emissions would be modestly lower by 2040, but that is actually an accomplishment: A growing economy, increased jet travel and wider consumer interest in big cars would tend to raise those numbers. Instead, the Obama administration’s regulations would push the economy to do more with less.

Unsurprisingly, the Rhodium Group, an economic consulting firm, concluded before the Obama administration reaffirmed the standards in January that they are an important part of the effort to meet the nation’s 2025 Paris climate commitment. “If the review process produced stronger standards . . . the action would help the US make modest additional progress in meeting the 2025 target. If the review were to weaken the current standard, it would set back emission reduction efforts considerably.” By reopening review, the Trump administration is likely to take the latter course.