U.S. environmental regulators proposed keeping intact tougher fuel-economy standards, intensifying a debate with car makers over one of President Barack Obama’s signature programs in the administration’s waning days. The Environmental Protection Agency on Wednesday proposed keeping future targets that call for auto makers to sell light vehicles averaging 54.5 miles a gallon, or roughly 40 mpg in real-world driving, by 2025. The proposal, originally planned for next year, raised the prospect that the EPA could complete the future mileage targets before President-elect Donald Trump takes office on Jan. 20. The proposal, now open to a 30-day public comment period, is part of a midterm evaluation of mileage targets that still requires a final determination from regulators originally set for 2018. Rules called for regulators to first propose whether the standards intended to cut noxious tailpipe emissions should be relaxed, toughened or left unchanged. But the April 2018 plan for a final determination is flexible and the EPA could move to complete the standards once the 30-day public comment period lapses. A senior EPA official on Wednesday said the agency’s administrator hasn’t found a reason to revise the standards. “There is no required, precise date on which the final determination needs to be done,” said Janet McCabe, acting assistant administrator for the EPA’s air and radiation office. She declined to comment on whether EPA Administrator Gina McCarthy planned to complete the rules before Mr. Trump takes office. Still, the incoming Trump administration could take another look at the standards. John Mashburn, a senior policy adviser to Mr. Trump, said earlier this month the incoming administration would conduct a “comprehensive review” of all regulations, including fuel-economy rules. He said the mileage regulations were originally meant to combat foreign-oil dependence as opposed to climate change. Auto makers have lobbied for relief from the climbing standards, contending that they will be difficult to meet amid lower gasoline prices that are sending shoppers flocking to less-efficient pickup trucks and sport-utility vehicles. The EPA released a draft technical report earlier this year predicting car companies would fall short and achieve 50.8 mpg — about 36 mpg in real-world driving — given current market conditions. But the agency added that manufacturers have a range of technologies at their disposal to reach more ambitious targets at affordable costs, including gasoline direct-injection and stop-and-go features. –Amy Harder contributed to this article. Write to Mike Spector at mike.spector@wsj.com (END) Dow Jones Newswires