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Aerodynamic drag below ~250mph is different that speeds up to ~750mpg (around the speed of sound). There are three kinds of drag (as I understand it): shape drag, surface drag, and detail drag. Shape drag is dominate for ground vehicles, and the the interaction with the ground is also important.
The wing section of an airplane is essentially a teardrop.
The physics of air is what defines the lowest drag shape. The front of the moving vehicle pushes the air outward from the stagnant point - up and down and to the sides. Then after the apex of that displacement has passed, the rate that the air can *cleanly* close back down together is fixed by two main factors: atmospheric pressure and the localized higher pressure caused by the displacement itself. The latter one changes with the speed of the vehicle (more or less) in a linear fashion up to about 250mph; and so the same shape works well for any speed up to that 250mph limit.
If the surfaces of the vehicle are at the proper angle, this keeps the air "attached" to the vehicle. Too steep an angle and the will be turbulence when the air is no longer "attached". Too shallow an angle, and there is an increase of surface drag.
I hope this helps.
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