Part of the problem is that it's not just about the engine but also aerodynamics. Some effort goes into aerodynamics on the top and sides, but other than a few cases like Ferrari cars are a disaster underneath (and even Ferrari is concerned with downforce, not efficiency).

I think his leg was being almost torn off by his buddy - there is no way that engine and chassis can reach 90 mpg at any speed as sold. If told "do it" and given an open budget I think it could be done with chassis aerodynamics and engine tuning for one speed. Not much other than the engine block might remain, and if it got 91 mpg at 45 mph it might only get 30 mpg at 35 mph or 55 mph.

The 70's oil crisis VW and such Fatman mentions may have been tuned to a particular EPA testing methodology, or might have been tuned to 55 mph alone. Maybe modern versions are tuned to do well at a broader speed range and EPA tests are less susceptible to being gamed. And of course emissions and safety rules these days are very expensive in efficiency.

I had a VW diesel Rabbit around 1980 and don't ever recall cracking 50 mpg.