The wing doesn't really do anything for a street car. It really is more cosmetic than anything else. Mustangs just don't look the same without them.
If your remember or are old enough, the wing on the Quattrovalvole, and few Countach left the factory without one, but it is however completely useless.
The Countach does not suffer from aerodynamic lift at high speeds, so the rear wing only adds drag, a lot of cost (it was still an expensive option at about US $ 5000) and most important, a lower top speed.
If you can afford one of those Supercars, you can dish out the extra money for the wing.
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