[ERTW]

I'd heard the rule of thumb that rear wheel covers reduce Cd by 10%. I tested this in CFD last night. Keep in mind that rotating wheels affect Cd, and create really complex flows. I also tested wheel discs.

first picture shows the side view. Cd of 0.253. LaFerrari greenhouse. This sports car is 69.7" wide, and 46.8" tall. Frontal area is 1.61 m^2. Total drag is around 50 lb at 108 kph.

I wanted to show where induced drag comes from. Any pressure across a surface creates a force perpendicular to that surface (called normal force). If the normal force is at an angle from vertical, there is a component in the X direction, which causes drag. Downforce and lift both cause drag.

second picture is the flow in and around the wheel/tire. It moves around the tire, corkscrews within the wheel well, and through/around the wheel. it looks really nasty.

Wheel covers. Cd 0.252. ONE count :/ compared to some VERY open wheels (not even modeling brake rotors)

wheel cover to the wheel centreline - gives you most of the benefit of a full wheel cover. The tops of the tires are moving against wind direction, and stir it up. The bottom of the tire is moving with air direction, and so are not as important. wheel discs plus half cover = 0.246. seven counts over baseline. still not impressive.

full wheel cover. Cd = .242 eleven counts over baseline. four counts lower than half cover.
Total percent difference = 4.3%
Fuel economy savings ~2.2%
An average car getting 30 mpg will see ~0.66 mpg increase at 108 kph.

discussion: wheel covers and discs shield the airflow from nasty wheel vortex...so it may not be such a bad analysis. Just the cover-less model will be somewhat higher drag. The more shielded the tires are in front view, the less they benefit from wheel covers. The GM Precept achieves a Cd 0.17 with half wheel covers and multispoke wheels.

note: I put a cone on the tail which tapered at a 16 deg included angle. it improved Cd 20 counts, but more than doubled lift. At least on this vehicle, this isn't an optimal solution.