I feel a little bit better about Avis after my sojourn with the rental vehicle completed.. Even though they tried to pull some shenanigans with the fuel tank upon rental, I still had a largely positive experience with them in retrospect.
Upon renting, I made a point that they write down the less than full gas tank on the contract, which they did with a good attitude.. It turned out I needed the car much longer than initially intended, and extending the rental was a breeze.
The car, a Passat, was the smoothest and quietest ride I'd ever had..and despite all the driving into the Rockies etc, I never even had to refill the tank. The trunk was huge too.
Upon returning the vehicle, it was duly noted that the gas tank was not full at rental time (even though it should have been) and they had no problem crediting me (since I always prepay for gas).
So all in all a bitter sweet experience