When I was a small child my parents had an Isetta for a while. In the late '70s I had a Honda an600 sedan. Worst tank was 38mpg and normal was 45. 600cc 2 cyl. here's a great vid of one.
In the mid '80s I had an '80 Audi 5000 diesel 5spd that my brother had bought new. No turbo but it cruised effortlessly on the highway at 85 and got 45mpg doing it. Huge fuel tank. The range was around 750 miles per tank if I remember correctly. I loved that car. I once drove from Indianapolis to Richmond on a tank of fuel no problem. This pic is not mine but it looked just like this.
'80 Audi 5000 diesel | Flickr - Photo Sharing!
With gas at $4 a gal I'd love to have both again.