

It's pretty complex, and Cd isn't the whole story either.  Cd for cars is
usually calculated based on the frontal area of the car.  So a large car
with a good Cd could get the same drag force as a smaller car with a
poorer Cd. 

To calculate drag use this formula:

D = 1/2 * rho * v^2 * Cd * S

Where D is the drag force (lbs), rho is the local air density (slugs/ft^3),
V is the velocity (ft/s), and S is the frontal area (ft^2).  Note that the
pieces called 1/2 * rho * v^2 are sometimes called qbar or dynamic pressure
(a fancy aero term for air pressure or force). 

Note that power is:

P = F * v

Where P is power (lbf-ft/s), F is the force, drag in this case (lbf) and v
is velocity (ft/s). 

Note that if you put the whole equation into one (by substituting D for
force) you get a velocity _cubed_ term.  That's why huge increases in power
result in little increases in speed.  Ditto for decreases in Cd. 

So if you have a 100 mph car and reduce Cd from .34 to .33, your new top
speed is:  (sound of trumpet fanfare)

101 mph

Sorry to dissappoint.

