I never ever get a truly full car. I think Avis et al. may have actually topped off the cars in the past. The needle usually shows full but will drop within a few miles. Thus, I usually return the car with the needle on F but 30-60 miles on an overfilled tank. On many GM cars a truly full tank will have the needle above the F mark, but I can't remember getting a car like that.

I used to rent mainly from National 5-6 years ago because one of their besgt features in addition to the Emerald Aisle was that you would be charged "market rate" for gas.