The drag coefficient measures how aerodynamic a shape, such as the body of a car, is – and therefore how energy-efficient the car will be when driving at speed. Some example values are shown below, listed from low-drag to high-drag. A solar-powered Hummer is probably not on the cards any time soon.
Entries hoping to win the World Solar Challenge Challenger class should be aiming at drag coefficients around 0.1. In the Cruiser class, values under 0.2 would be appropriate. This year, the wide range of car shapes in the World Solar Challenge demonstrates that there are many ways of achieving these goals.