Might be off-topic but if I pick up a car with less than a full-tank, I will take picture with my smartphone of the gauge with the mileage reading. I would use this as proof that I picked up the car with a less than full tank.

I am a hertz loyal customer but I like how Avis will give you credit if you actually returned the car with more gas (as indicated by the fuel gauge). There have been times that I actually made more $$ by pickup a car with less gas and filling it up myself. However, there are alot of more things wrong about Avis that keeps me at Hertz.