It's no surprise that some of the most economical cars that have gone on sale have also been some of the most aerodynamic--the two go hand-in-hand.
Light weight helps too, and a small engine that sips as little gas as possible. The best example of modern times would be the first-generation Honda Insight--under 2,000 lbs at the curb, a tiny 1.0-liter 3-cylinder engine, a 0.25 coefficient of drag, and a small frontal area.
These ideas are nothing new, of course, and to prove it, we've found a rather special record vehicle from 1981.
Based on the Citroen 2CV--to France what the Beetle was to Germany and the Mini to England--it swaps the flimsy steel bodywork of the regular production car for an incredibly sleek, aerodynamic shell. This was probably easier than it sounds, as like the Volkswagen Beetle, the 2CV used a separate chassis to which the body was bolted.
1981 Citroen economy car by Luigi Colani (Image: Colani Design)
No coefficient of drag data is available, but the net result of the streamlined body was a world economy record back in 1981--1.7 liters of gas per 100 kilometers. In real money, that's an incredible 138 miles per gallon.
Subsequent economy record holders have been more purpose designed, but Colani's design showed what was possible using essentially standard car mechanicals.
And while the shape couldn't really be considered practical for road use, it's illustrative of just how important good aerodynamics are to fuel efficiency.