the Shell Eco Marathon proved that ridiculously high mpg numbers can be attained

1950 Opel 376 mpg and it was done in 1973, with a 23 year old car. The mileage competition was over a 10 mile course with a minimum speed of 10 mph average enforced. This Opel is now in Seattle Washington

In 1939, a group of Shell scientists based in a research laboratory in Wood River, Illinois, USA, had a friendly bet to see who could drive their own car furthest on one gallon of fuel. The winner managed a distance of 21.12 km/L (59.7 mpg‑imp; 49.7 mpg‑US). A repeat of the challenge yielded dramatically improved results over the years:

149.95 MPG with a 1947 Studebaker in 1949
244.35 MPG with a 1959 Fiat 600 in 1968
376.59 MPG with a 1959 Opel in 1973.
https://en.wikipedia.org/wiki/Shell_Eco-marathon

1924 Chevy 168.47 MPG, this is now in the Wood River Museum in Roxana Illinois where they used to have the Shell garage that had the contest

244 MPG Fiat 600

The 1968 Shell Mileage Marathon Fiat achieved a new fuel mileage record of 244mpg. It was then stored and later moved in the 1980's  to Talladega Raceway park where it was found in 2005, weathered and deteriorated

http://carzhunt.blogspot.com/2015/01/so-we-cant-get-high-mpghuh-part-1.html
http://www.shell.com/energy-and-innovation/shell-ecomarathon.html
http://www.59fiattestcar.com/