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Hi Bob Wilson,
My guestimate is that the AirTabs are just too big for the boundary layer thickness on cars. They were designed for 18 wheel truck trailers - which have very stagnant flows over the rear third of the trailer. Driving next to 18 wheelers is a bit of a buffeting experience.
Another poster on here found that just a .030 inch thick Dymo tape, cut into sailplane style turbulators was effective in causing the airflow sideways off a windshield to settle down and flow down the side of the car, when placed on the upper third of the A pilar. The idea is that the turbulator converts the flow momentum into turbulence - which is good for causing the sideways flow to turn and flow with the prevailing flow on the side of the car.
In sailplanes, turbulators are used to improve where the seperation point occurs on the wings (further back is better, of course). This seems to be a similar problem to the flow seperation on a car. Except the boundary layer on a car is much greater than on a thin sail plane wing.
The flow off the top of the car has momentum, and cannot bend downwards because of the momentum of the air quickly as the flow needs to conform to the shape of your car. The boundary layer on top of the car is probably about 1/4 to 1/2 inch. So, try some turbulator tapes that are 1/16 to 1/8 inch thick. This might result in reducing the momentum of the flow enough that it will follow the shape of the car. I have fabricated thicker turbulator tapes out of FOAMIES foamed plastic from crafts stores. Some is avilable with a self-adhesive backing. Its available in a wide variety of thicknesses - ideal for experimenting. The zig zag pattern can be cut with Pinking sheers.
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