Thanks for everyones input. Hopefully I will sell the thing before it needs new tires. I only drive it about 3,000 miles a year, so it is a good bet, but if not, I'll be going with a highway tread for sure.
I had a nice 3-hour flight today with full view of airtabs in action.
For those aviation fans our there, here's the aircraft: the Boeing 757-200, 5500-Series on NWA. (757-251)
As I looked down the wing at these 14 items, it became perfectly clear. Unless you plan on using the top of your vehicle as an airfoil, and you're concerned about losing lift when going up hill, then forget about it.
The link that Treb' posted made it plain as day. In aircraft, it holds the air to the top of the wing, using vertices -- shaking up the airflow so it "sticks" to the top, and the wing doesn't loose lift when tilted upward.
What advantage does this have in a car? It shakes up the air, and creates a disturbance at the rear of the vehicle -- an unwanted side effect of the average vehicle anyway. Mitsubishi did a test of it on the Evo-9, and came to an interesting conclusion (you'll still find it on the car, though).