The drag coefficient (Cd, Cx or Cw, depending on the country) is a dimensionless quantity that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. Two objects of the same frontal area moving at the same speed through a fluid will experience a drag force proportional to their Cd numbers. Coefficients for rough unstreamlined objects can be 1 or more, for smooth object much less.
Cd in automobiles:
(..) Minimizing drag 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. Indeed, aerodynamic drag increases with the square of speed.
Now I'm really lost. I took Principles of Fluids Dynamics (or whatever you call it in English) when I was at the University and I don't get why frontal area and velocity wouldn't count when calculating Cd.
Here's more data: http://www.grc.nasa.gov/WWW/K-12/airplane/dragco.html
The drag equation Cd = D / (A * .5 * r * V^2)
and how it's measured can be found there.
Is there anybody to explain it to me?