Trik: those look fun, but I still like this modded smart:
Tandem seating a la VW 1 liter car means both reduced frontal area and lower Cd.
Escaping from the original car's poor aerodynamics to this shape results in 2.5 L/100 km / 94 mpg US / 113 mpg Imperial.
Based on the previous gen smart (with the diesel):
Modded smart car: Jetcar 2.5