Drag coefficient
The drag coefficient is a number that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. Different objects with the same drag coefficient will behave in similar ways, after scaling for differences in size.
A cylinder is given a default drag coefficient of one. That means that two cylinders of the same size will have the same drag, one twice as large will have twice the drag. Less streamlined shapes will have higher values, while smoother shapes will have lower values.
Automobile Aerodynamics
The drag coefficient ("Cd") is a common metric in automobile design, where designers strive to achieve the lowest possible drag coefficient, and thus lowest possible drag.
This is done to improve fuel efficiency at highway speeds, where aerodynamic effects represent a substantial fraction of the energy needed to keep the car moving. It is also a factor in sports car design, where low drag, coupled with reduced lift, results in a car which can achieve stability and high top speeds.
The typical modern automobile achieves a drag coefficient of between 0.30 and 0.35. SUV's, with their larger, flatter shapes, typically achieve a Cd of 0.35-0.45. Sports cars can achieve figures of 0.25-0.30, although sometimes designers deliberately compromise drag, in favor of reducing lift.