[SOUL-DRIFTER]

Here is an interesting article on wind drag...
Re: Does a well waxed automobile get measurably better mileage than unwaxed?

•from Clarence Chang, Ph.D., NASA Lewis Research Center, Cleveland, Ohio:
Below 35 mph (there about) the skin drag is dominant. Above that speed, the pressure drag dominates. Skin drag is linear with speed. Pressure drag is proportional to speed square. Polishing the painted surface won't do anything for the pressure drag, only the skin drag.
However, the turbulent drag from the various protrusions are much more dominant than the skin drag. For example, if you enclose the bottom of the car in a smooth flat skin (that is, no exhaust pipe and other stuff exposed), you can easily get the drag coefficient to drop another 20%. The drag coefficient on a typical car is about 0.35. 20% of that is alot. If you open the windows, for example, you can easily increase the drag 5 to 10% or so.
Making the surface smooth only works for bodies with very low Cd, say in the neighborhood of below 0.1. There are several easy ways to reduce drag. You can enclose the bottom of cars with a smooth skin, but of course the shops don't like it since you'd have to take the panels off to work on the car. If however, you lower the body so that the distance between the floor pan and the ground is closer, drag goes down too. You can also put an air dam in the front of the car to route the air around instead of letting it go under the car. That works too.