Double your speed and your drag increases FOUR FOLD.
Sticking a wing, spoiler, etc on the rear of a car is an attempt to make up for a aero design deficiency. A perfect example of good aerodynamic design (not perfect design) is the soap box derby racer bodies (do a Google Image search). Notice no spoiler, wing, etc.
Lift, whether negative lift or positive, creates drag. And a wing at zero angle of attack still produces parasitic drag, which increases 4X as speed 2X's.
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