The only way they are feasible is by downsizing. I have a 67 Camaro (which is admittedly a gas hog) so I have been trying every trick to get mileage up. The Camaros drag coefficient is actually pretty good, and modern cars have close to maxed out on mileage gains there. Changing the transmission helps a lot, but Chevy already has a 6 speed automatic so they’ve gotten the gains there. Electronic fuel injection and computer metering have already been standard.
So, you are really left with downsizing in weight and cross section. I don’t think people will be happy, they will all keep their old cars.
As a fellow 67 owner, I’m surprised you’d drive it enough miles to even worry about the fuel economy.
That’s the economics of government intervention. Make it costly to do what you want and people will find a work around.
Bastiat was right.