We’ve all been at the gas station, filling up our tanks, and the pump shuts off, just a few cents short of an even dollar. If you’re like many other car owners, you most likely add a bit more gas to round the price to an even dollar.

While this simple task may seem harmless, is it really something you should do? The answer: No. You should never top off your vehicle’s gas tank and our auto repair experts explain why.

Editor’s Note: This blog was originally published in 2012 and has been updated in April of 2021 to reflect technical and industry changes.

Why Shouldn’t I Top Off My Gas Tank?

Your gas nozzle is designed to automatically click off for a reason—to alert you that your tank is full. When you top your gas tank off, the gas reverses, flowing directly into the station’s tank, because your gas tank is already full. Your tank doesn’t get any fuller, and your wallet only gets emptier.

Your gas needs room to expand inside your gas tank. When you top off your gas, you leave little or no room for gas to expand, causing the gas to collect in the vapor collection system of your car. If the system fails to function properly, your car can have high gas emissions as a result.

Gas station pumps today are designed with vapor recovery systems. These preventative systems stop gas vapors from escaping and affecting air pollution levels. The station’s vapor recovery system can, however, operate incorrectly or fail entirely when you top off your gas—further polluting the environment and negating the implementation of these recovery systems.

Topping off your gas tank can also cause various amounts of pressure to build in your car’s gas tank, which can lead to flooding the carbon filter vapor collection system, which is only meant for vapor. This is no joke – and we have seen many cars at the shop with bad vapor canisters because people top off their gas tank.

About Christian Brothers Automotive:

If you are a repeat offender of topping off your gas tank, your check engine light might come on to indicate a problem in your car’s major (or minor) operating systems.

From routine maintenance to check engine light diagnostics & repairs, our team offers reliable auto services you need to keep your vehicle safe on the road.

At Christian Brothers Automotive, we’re here to help you navigate through the complexities of auto repair and maintenance. We offer a no-obligation courtesy inspection, shuttle service, and leading-industry warranty on parts & repairs.

We welcome you to find your local Christian Brothers Automotive store by using our store locator here.