What happens when you prepay for example $25, as opposed to filling up? When you fill up your entire tank, you hear it pouring out gas until your tank is full and then it stops, but when you prepay $25 for example, you hear it pouring out gas until around $24.67 (by Shell for example), and then you don't hear anything as it just slowly counts down the rest of the pennies.
Are you really getting gas for those 35 cents, or is it just counting out those pennies to hit $25?