Prepaid gas plans are almost never cheaper, so fill up on your way back to the drop-off location. However, you should try not to fill up at the closest gas station to the rerun spot, unless you're renting from a small town in the middle of nowhere. Gas stations in cities and close to airports are always more expensive because of increased demand. I recommend downloading an app like GasBuddy to find the cheapest gas in your area, then using a credit card that gives rewards for gas purchases.