Transportation Secretary Sean Duffy used X on Saturday to preview a Monday announcement on corporate average fuel economy standards, Associated Press reporting says. President Trump amplified the message on Truth Social, casting looser mileage rules as a way to cut “waste” in carbuilding and lower sticker prices. NHTSA’s December projection sketched a fleetwide average near 34.5 mpg in model year 2031 versus about 50.4 mpg under the prior path.

CAFE is administered at Transportation/NHTSA even when EPA tailpipe rules move on a parallel track. That split matters for litigators: challenges often target the record NHTSA compiles on feasibility, consumer cost, and energy effects. Environmental groups including the Sierra Club and the Center for Biological Diversity already promised a fight, arguing weaker standards raise pump costs and pollution while gasoline sits near $4.47 nationally on AAA’s Sunday print.

Automakers have publicly favored rules that preserve affordable gasoline models. Whether Monday’s final text matches the December projection — or softens further — will drive both equity reactions in Detroit names and the first wave of NGO lawsuits. Read the Federal Register notice, not only the social posts, before treating any mpg number as final.

AP reporting and Duffy’s Saturday preview agree Monday is the CAFE announcement day. The administration sells affordability and production; green groups say the rewrite raises fuel burn and pollution.

Transportation Secretary Sean Duffy previewed Monday’s fuel-economy announcement on social media.

The package is expected to lock in a much softer mpg path than Biden-era rules on the Monday wire.