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Vortex generators increase induced drag, but have the potential to reduce from drag. The net effect can be positive, but lower overall drag would be very hard to achieve by trivial placement as even on airfoils, which are much easier to work with than the back of a car, the effectiveness of vortex generators is highly dependent on chord placement, vg angle of attack and vg density, and you would not position them at the same place wheter your goal is drag reduction or, as in the case of an airfoil, lift.
I don't see what good they could do right before the wake break point.
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