The Fiat Volugrafo, 3.3l/100km in 1946?
 

Barely a car!

Fiat's one cylinder Volugrafo a streamlined auto that is - Photo - LIFE

Video of some guys trying to get one running. Some nice shots of the inside and a bit of the engine.

http://www.youtube.com/watch?v=7hm2jQf6fGw

I hope this link works, it has more pictures and information, translated from spanish to english.
Translation result for http://www.autopasion18.com/HISTORIA-VOLUGRAFO.htm

If it isn't working, the link is VOLUGRAFO HISTORIA

I searched the site to see if it was posted before, I think it's neat.

Nice!
Imagine it with an aero-roof, different headlights and windshield!
I guess the low consumption is from small weight, small frontal area, and low speed. Great for city driving, but not in SUV-land.