Five environmental groups take the 34.9-mpg fuel economy rollback to a federal appeals court

The 2031 target is now 15.5 mpg below the Biden-era projection ^[16], a cut of about 31 percent ^[17]. Anyone setting the efficiency content of that model year has two numbers to weigh: the one in the new rule and the one it replaced ^[5].

The petitioners want the rule halted. "We're asking the courts to put a stop to this callous giveaway to Big Oil and Big Auto," David Pettit, an attorney at the Center for Biological Diversity's Climate Law Institute, said in a statement ^[7]. Katherine Garcia, director of the Sierra Club's Clean Transportation for All program, made the legal argument directly. "It is unlawful for Trump to turn back the clock on fuel-efficient cars, forcing drivers to waste more money on gas and communities to breathe toxic air," she said ^[6]. AP's account does not say which circuit will hear the petition or whether the groups have sought a stay ^[1].

The administration is also arguing on legality. Transportation Secretary Sean Duffy said the new rule cuts the "illegal mandate that forced automakers to produce more expensive electric vehicles that American families didn't want" ^[9]. According to AP, there has never been a mandate that manufacturers sell EVs or that Americans buy them; the earlier standards pushed automakers toward a mix of electric, hybrid and more efficient gasoline vehicles ^[13]. The administration also said the change would give buyers more choice of vehicle and move them into newer, safer cars ^[8]. The Transportation Department and NHTSA did not immediately respond to AP's request for comment ^[10].

The case for planning to the looser number comes from automakers. Auto companies and industry groups broadly welcomed the change, saying the Biden rules were too stringent and did not benefit consumers ^[12]. Building to that preference is a bet that the rule survives review. The trade-off is direct. An automaker that cuts efficiency work to match 34.9 mpg keeps the savings if the rule holds, and has to rebuild that work with less time before 2031 if it falls ^[5]. One that keeps the work pays now for engineering it may never need.

This week the 2031 standard is 34.9 mpg ^[5]. The exposure is in this year's budgets, where spending on 2031 efficiency is set against a target now before an appeals court ^[1].