Just had a related experience at LAS. Picked it up and noted it was less than full, but no way I was going to get back in the monster line to get another car. Drove less than 50 miles before returning, and stopped refilling when I hit 6 gallons at the gas station.

Upon return, I asked for a refund for the extra gas and they were going to process it....until they saw it was a Priceline rental. I was told that they can only charge your credit card for incidental charges, but can't give incidental credits if it's not booked through Avis. I saved $80 through Priceline, so I guess I still came out somewhat ahead.