
    I don't remember the formula's off hand as it has been awhile since
I took aerodynamics and haven't used the stuff since. 
The Cd is related to the drag force which is what effects top speed and
fuel consumption. When the drag force on the car or aircraft is greater
than what the vehicle's engine can overcome it has reached its top speed.
(autos of course also have to overcome rolling resistance)
Since drag opposes the vehicle's motion, the engine must make up for that
by burning more fuel. Anyway, since the geometery of an auto is rather
complex, the Drag,pressure coeffiecent,etc is either found experimentaly, or
using a numerical method.

anyway for flow around a cylinder the drag coeff is:

            Cd = d/(q*2R)

       where d is the drag force, q the flow velocity and R is the radius
of the clyinder.
   To get a rough (very rough) estimate, you can set R at 1/2 the car's
width, q at the car's speed and knowing the Cd, find the drag force that
the car would need to overcome. 
