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As I keep trying to point out when I see VG Gimmicks discussed, Skin drag is not the enemy of fuel efficiency with vehicles, it is pressure/form drag that we must address.
By messing with air only an inch or so away from the skin of the car, air which is already decidedly non-laminar...meaning it is already vortexing...these devices accomplish nothing.
If you want to improve your cars aerodynamics, you need to make changes in the shape of the car, this will affect the pressure, hopefully reducing drag.
Buffeting the buffeted air 1" from the skin of a car which is creating effects on the air many feet out from the skin in every direction is like wondering why the flow of a 6 foot deep river isn't changed by more than a tiny fraction if there is a stony bottom or a smooth one.
When you look at VG's in the big picture, they're a joke. You will never gain a discernible change in fuel efficiency with VG's alone.
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