No, you should not top off your gas

Gas pumps are designed to stop the flow of gas once your car’s tank has filled up. But as many drivers know, you can pull the trigger and pump a little more gas before the pump will click off again.

There are many reasons why you might “top off” the tank: Maybe you’re convinced your car has room for a little more gas, or it could be because you want to get the price of your gas to a nice, round number.

People online have debated if it can hurt you and your car or if it’s actually harmless. 

THE QUESTION

Should you top off your gas tank after the pump stops?

THE SOURCES

• Environmental Protection Agency (EPA)
• GasBuddy, a gas price information app
• AAA
• Santa Barbara County Air Pollution Control District

THE ANSWER

No, you should not top off your gas tank after the pump stops.

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WHAT WE FOUND

Gas pumps automatically shut off for a reason. After the nozzle first shuts off or clicks, pumping additional gas into the tank could damage your car and create a health and safety risk for you and others.

An Environmental Protection Agency (EPA) press release explains that a gas pump knows when to shut off because of a sensing hole located at the tip of the pump’s nozzle. This sensing hole is attached to a tube that sucks in air. When the tube is sucking in air, the gas pump’s valve stays open, and the nozzle dispenses gas. But once gas has covered the hole and the tube stops sucking in air, which happens when the gas tank is full, the valve closes and the gas supply shuts off. You can see a more detailed explanation of how it works in this video.

One consequence of overfilling your car’s tank is that it can harm your car’s evaporative emission system. This system is designed to burn off excess gasoline vapor. But overfilling your gas tank could cause liquid fuel to overflow and damage the system, GasBuddy says. 

Damage to the evaporative emission system can hurt your car’s performance and make your check engine light turn on. Replacing the damaged part can cost anywhere from $200 to $1,500, GasBuddy says.

Normally, the evaporative emission system will last for the lifetime of your car, AAA says. But topping off your gas tank even once runs the risk of filling the tank to the point that you ruin the delicate filters in the system, according to AAA.

In addition to causing damage to your car, topping off can damage the gas pump in a way that makes it more likely for gas to spill out of your car or the nozzle, GasBuddy says. Even if you don’t damage the pump, the gasoline can get stuck in the nozzle when you overfill your tank, causing the gasoline to spill out onto the next person that grabs it, the Santa Barbara County Air Pollution Control District says. 

Gasoline is bad for both human health and a fire hazard, on top of being an environmental hazard.