As we have seen with the Obama measures, some "efficiency regulation" can be gamed.

Need to look at the details to see, perhaps this will drive American cars to be smaller.

Tightening the "Light Truck" Definition: Starting in model year 2030, the National Highway Traffic Safety Administration (NHTSA) is amending the fleet classification system. The rules make it structurally more difficult for automakers to use specific design elements and equipment additions to trick the system into classifying small SUVs and crossovers as light trucks. This forces more of these high-volume vehicles back into the stricter passenger car regulatory bucket.

Elimination of Credit Trading: Historically, automakers who failed to meet efficiency metrics could purchase regulatory credits from cleaner automakers (like Tesla). The new regulation eliminates the inter-manufacturer credit trading system starting in model year 2028. This forces every manufacturer to stand on their own fleet's merits rather than subsidizing non-compliance through competitors