In 1979 I drove a car that got an honest 43 MPG.
It ran on leaded fuel, no cat, had a carburetor and points.
It got my *** from point A to point B reliably for over 10 years with one engine rebuild.
The technology isn't the issue, it's the buyers. Sure, the auto manufacturers could probably build a 100 mpg vehicle, but then they have to sell it to someone and we are in the middle (or maybe end?) of another horsepower war. I bet even the guy shopping for the cheapest 4 cylinder hooptie is looking at the HP specs and influenced by the 0-60 time compared to the other cars in the same class. I know I would be.