[NeilBlanchard]

I am "taking that into account". Lowering a car too much actually increases the Cd because there simply isn't room for the air to move around, and with a typical rough and lumpy and crevassed underside, this can cause some additional drag. You want the air to flow easily and smoothly, and moving vehicle is "squishing" the air around and the lower the vehicle, the less space there is for this to happen.

With a smooth underside then air is not as turbulent and the reduction in Cd is way greater than the reduction in area from lowering. Ideally, the rear of the underside is a slightly rising slope, and the ideal minimum is about 6-8" of clearance. Less clearance compresses the air more and causes lift - which is drag. A air dam adds frontal area and increases drag; though it reduces lift or even adds downforce - which is also drag. A smooth underside negates the need for an air dam and it reduces turbulence.

A smooth belly pan will be a much bigger improvement than lowering the car.

Edit: to emphasize GRU's point, look at the Schlörwagen. In the Hucho book, they have a chart showing the Cd at various heights - the *higher* it was the Cd went down (but this adds to the frontal area, and there was exponentially diminishing returns above a certain height). Lowering the car added Cd - more so with a rough underside. The best compromise with a smooth underside was with about 6" of minimum clearance.