I know all too well how these rent-a-car places get you for gas.
Last fall, I had an accident with a rental car in CR. They sent a flat bed to bring me a replacement car and take the damaged one away.
When I returned the new car at the airport before going home, I was told I had a $48 gas charge because the first (damaged) car was returned with 1/2 tank less than it went out.
I suppose I could have asked the nice tow truck driver to stop at the gas station so I could fill up the busted car that was sitting on his flatbed. MUY TICO!
But I realized that I was in CR and haggled them down to $24 which is a fair price for a 1/2 tank.
Pre-paying gas makes no sense in most situations. I've had occasions where I rented a car for 3-4 days and I didn't use the whole tank, maybe half. So why pay for a full tank and only use half?
The only way it would work is if you were given a car with 100% full tank, and you brought it back bone dry, then you got your money's worth. If you were given any less than a 100% full tank, or you brought it back with any gas in the tank, you are losing money (aka they are robbing you). I assume they probably give you a 7/8 or 3/4 full tank knowing that they are making more money the less gas you start with.