Filling up the gas tank all the way each time you visit the gas station is the most fuel-efficient way to fill up, since it means you'll make fewer trips to the gas station over time. But avoid the temptation to "top off" after the pump automatically stops. The reason is simple: Adding more gas wastes money. Gasoline tanks have something called a vapor recovery system, which prevents gas from escaping. Adding more gas means you add some gas into your tank after it's full, but you also send gas back into the tank. That's money down the drain!