Topping off your tank
Myth: Many people continue to add gas to their tanks even after the pump has indicated that the tank is full. By topping off their tanks, they believe they're getting as much gas in the tank as possible.
Fact: In reality, topping off can actually send gas back to the station instead of into your car. The gas nozzle automatically shuts off when your tank is full, so any additional gas you try to add will be drawn into a vapor recovery system and back into the station's storage tanks. In addition you may damage you car's evaporative emissions system.