FWIW, I put a couple of VGs on the roof of my Audi 80 sedan, and they did a nice job keeping the air attached to the back window. The water drops sitting there were bugging me, but with the VGs, the airflow would clear them off. They were about a foot ahead of the roof/window junction to give the vortex some room to form and attach to the surface. I did at least find duct tape that matched the color of my car, though (white).
I also noticed that the overall airflow across the rear window and rear quarter panels seemed to converge more toward the centerline of the car. unfortunately I drive too erratically to notice whether the economy was better.
I eventually took them off because they were on with duct tape and I didn't want it to get too crusty. And I was tired of explaining them to people. My wife dubbed my car "Sharkey" when she saw them, and I'm afraid the name is going to stick.
