I agree with Baze.

"Prepaid gas" is a gamble .. how dry can you bring the car in?

A long time ago, Alamo used to play this game on every rental. They would look at how much gas was in the tank when they gave you the car by estimating the number of gallons in the tank (as brought in by the last person), and charge you for that much gas at the time of the rental (example: 16 gallon tank 1/4 full, Alamo would charge you for 4 gallons) and then explain to you at the time of rental and on the contract that you should bring the car back empty. Of course, it's impossible to bring the car back empty, so they end up charging the next customer for the 1/4 - 1/2 tank people would bring in every time, except on the very rare occaision that somebody would actually bring the car in completely empty. That practice was so sleazy along with all the rest of their practices they eventually had to change them. (I vaguely remember getting the infamous Alamo hard-sell insurance pitch on a "free" florida fly-drive package from one of the airlines a long, long time ago.)

If you aren't renting from National, you should always decline the prepaid option and gas the car up yourself on the last day before returning it.

-David