I do not believe any car ever achieved 70 mpg, and certainly not in mixed city and highway driving. Even the 3-cylinder 5-speed Geo Metro, which was legendary for its efficiency did not come close to this.
The first car I ever owned was a 1988 Toyota Corolla, with the carbureted engine and 3-speed automatic. With careful driving, it could achieve about 40 mpg, which I thought was very efficient. I have owned several vehicles since then, and none have crested the 30 mpg mark, and I try to drive as efficiently as possible.
I think it would be possible with current technology to build a lightweight (by today’s standards) car with a 100 to 120 hp 4-cylinder engine and 4-speed automatic transmission, which could achieve 40-50 mpg in mixed city and highway driving. Of course, there would be very little market for such a car in the U.S. however in other markets, people would want such a vehicle.