[MetroMPG]

I recall reading a comment posted by an aerodynamics expert working for an automaker. I'll see if I can find it.

It boiled down to: while Cd does vary with airspeed, the variation is small enough in the range of speeds a road vehicle typically drives at that it's not worth worrying about. (I seem to remember -- but will check for it -- that he was talking in terms of thousandths - eg. 0.00x).