Thoom! You’re at the gas pumps and you hear this familiar, sonorous shut-off click signaling that your tank is full. The total reads $20.41 and that odd amount of change due somehow feels like wearing one shoe.

You decide to squeeze the nozzle to get that extra 4 cents for a comfortable multiple of 5, or if you have a great discount from shopper’s rewards you may even try for an extra 50 cents- despite the warning label that advises against topping off. It’s likely that you wait until your car is riding on fumes before you fuel up so you can get that great discount per gallon.

Before you go beyond the pump’s automatic cut off, you should know why topping off your fuel tank is not such a great idea.

Topping off can cause damage to your fuel system

Your car is engineered with a vapor collection system that can be compromised by overfilling its fuel tank. An overflow could cause liquid gas to enter the charcoal canister, or carbon filter, which is designed only for vapor. Not only will this shorten the life of the canister, but the backflow will eventually damage your engine.

Also, gas expands as its temperature increases. Even if you have a vented cap or a vent line to relieve vacuum and pressure from the tank, a surplus of fuel for the size of your tank will not leave room for the fuel to expand and could cause a serious consequence.

Topping off harms air quality and your health

Gas pumps are designed with a vapor recovery system. Once the gas tank is full, the system is designed to take back vapors and surplus fuel to prevent them from escaping into the atmosphere. Additionally, if you pump enough fuel to run over it brings on a double whammy. Loose vapors increase ozone levels and cause irritation to your airways, plus higher ozone irritates your respiratory system.

For these reasons, the EPA enforces the pump shut off mechanism in an attempt to reduce detrimental effects to human health and our environment.

That great deal will become void

That extra few cents you tried to pack into your fuel tank will only wind up back in the gas station’s storage well. The gas pump’s vapor recovery system will just take back the extra fuel you pumped to avoid harmful environmental effects and a possible explosion.

And that damaged charcoal canister won’t be a cinch or cheap to repair once it becomes damaged from topping off. Depending on the make and type of vehicle you drive, a new canister plus the labor could run in the ballpark of $1000 dollars! The 50 cents per gallon discount suddenly doesn’t look so attractive. Hopefully, you will be spared on damage to the gas take or engine.

It’s best for you, your car and the environment if you accept the automatic pump shut off as your tank’s message that enough is enough. An uneven amount of change due in your fuel total won’t hurt anything – it’s just a number!