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A car is three dimensional, so the 2D profile of an airfoil is only part of the picture. the sides should be tapered -- in fact, the sides can be tapered about as much as the top, and since the taper happens on both sides, this would shortened the length quite a bit, and end up forming a "fish tail".
Look at the Edison2 VLC (Very Light Car) for an example of this. The VLC was tested in the GM wind tunnel and got a Cd (using the older SAE formulas) of 0.145; so this is comparable with most of the Cd numbers we have used here on EM. The newer SAE formulas gave a Cd of 0.164. The VLC has a relatively "pointy" front end and outboard wheels, so if you apply the above profile to a chassis that has enclosed wheels (and a much narrower rear wheel track), then the Cd could likely be even lower.
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