I am not sure whether your gas mileage numbers for these old cars are right.

One fundamental difference between the cars of the 80s and today is the difference in horsepower, which has been steadily increased (eliminating gains in fuel economy). It's telling (and somewhat silly) that Stevenson thinks that 70 is so underpowered - I drove a subcompact (4 seater) in Europe during the mid 90s with 50 hp, and never felt scared/underpowered (even on the no speed limit Autobahn).