From the article: Ok, leaving away this technical explanation (yes, I'm a car technician, what you want me to do :P) i'll show you some little tricks that make some reduction of the aerodynamic force that goes against your car.

that's a little guide that shows the amount of gain or losing aerodynamic force. Gain means, more gas spending, and losing force, less gas spending

lowering the car 30mm -5%
flat wheel covers -1 to -3%
wide rims + 2 to +4%
windows at the same level of the body -1%
filling gaps (doors, windows...) -2 to -5%
underbody shape (as flat as possible) -1 to -7%
retractable lights + 3 to +10%
side mirrors +2 to +5%
air flow through the engine bay and
radiator +4 to +14%
brake ventilation +2 to +5%
ventilation of the interior +1%
open windows +5% (notice that the air conditioning,
increase more the dragging force of the car
against him, so is preferible to go with
open windows instead of A/C)
open roof + 2
surf board in roof +40%

Not the article I expected from the pin, but interesting breakdown of aerodynamic drag on a car. Couldn't care less about the MPGs, but less drag = more speed in my books