Originally Posted by jsr72
To produce an identical level of aerodynamic form drag, if the air density is increased then the velocity must correspondingly decrease.
In highly idealized situation if the air pressure happens to be the same at two different temperatures, then the new velocity is simply the original velocity times the square root of the new temperature divided by the square root of the original temperature. (Use absolute temperatures in the calculation)
For instance, if a vehicle is traveling 70 mph at 70F, then it experiences the same amount of form drag at 67.3 mph at 30F.
I think the ideal case is pretty good - without putting the car in a wind tunnel and checking it.