Even if the "nanny-state EPA" gets gutted and neutered as the current administration desires, I don't see us going back to carbs and shedding catalytic converters anytime soon. I expect "market forces" from abroad to keep US car makers thinking about fuel economy and emissions. Just on a personal level, think how car exhaust isn't nearly as smelly as it used to be. Customers are used to cleaner-smelling cars now.
Best tank (so far): 32 MPG