The give you a discount on the prepaid option to entice you because they are likely to come out ahead. You (usually) pay for the whole tank, no refunds for unused fuel at return. The only real way to come out ahead (depending on the value you place on convenience) is to time it out so the car is literally stalling out as you coast across the tire shredders into returns. Chances are you will return the car with at least a few gallons of fuel (that you paid for) in it for them to re-use. This, combined with the fact that National buys gas wholesale mean that the prepaid fuel is a huge money maker for National.