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Is It Okay to Top Off Your Gas Tank?

October 25, 2010

For as long as I have been driving, I have topped off my gas tank when refueling. I think I started doing it so I could be sure I had filled up my car all the way to the brim because refueling was always kind of annoying. I also remember topping off the tank so I could round off to an even dollar amount—for some reason, $26.00 seemed much better than $25.47.

The other day though, I was filling up my car and when the pump clicked, I removed the nozzle, put it back in its holder, and went on my way. After nearly twenty years of driving, why would I suddenly change my mind?

Well, for one thing, who cares about having an even dollar amount? But the main reason I stopped is because I remember reading somewhere that it’s not a good idea to top off the tank when refueling. So I have decided to do my own research on the matter—here’s what I have found out.

Basically, topping off your gas tank is a bad idea—both for the environment and your piggy bank.

Bad for the wallet
When you are refueling, the gas nozzle clicks for a reason—because your tank is full. If you keep pouring fuel into an already-full tank, where is the excess going to go?

In areas of the country with ozone nonattainment (where air pollution levels persistently exceed the national ambient air quality standards), pumps are equipped with vapor recovery systems that are designed to prevent vapors from escaping into the air and contributing to air pollution. These systems feed excess gas back into the gas station’s storage tanks. Therefore, if you pump extra gas into your system, you are actually paying for the gas that is fed back into the gas station’s tanks—and for the gas that spilled onto the ground.

Another reason topping off is bad for the bank is because extra gas may evaporate into your vehicle’s vapor recovery system. Gasoline needs to expand inside the tank, which is why a vehicle has a vapor recovery system in the first place.

In the old days, gas caps used to have holes in them so that vapors could escape, which seemed like a great idea until scientists realized the vapors were escaping into the atmosphere and causing smog and awful things like lung disease. The EPA aimed to solve the problem by requiring all vehicles to have a vapor recovery system.

A component of the vapor recovery system is a charcoal canister, which is attached to the gas tank. When excess gas vapors are created, they are absorbed by the charcoal and held there until the next time the car starts. When the car is next started, the vapors are sucked back into the intake air and, during the combustion process, are burned by the engine. If liquid gasoline is forced back into the charcoal canister from topping off, the canister will be ruined, which costs about $300.

Bad for the environment

It’s no secret that gasoline vapors stink, but they are also harmful to breathe. The vapors contribute to the ozone and are a source of toxic pollutants like benzene, a known carcinogen. In Oregon, for example, officials discovered that the state’s gasoline contained incredibly high levels of benzene and ultimately passed laws against topping off in order to reduce the state’s carbon emissions, which were among the highest in the country.

Topping off can also cause the gas station’s vapor recovery system to operate improperly, which contributes to air pollution and can cause the gas pump to stop working. It also leads to higher emissions, poor mileage, and decreased engine performance. In addition to simply wasting gas (and money), these problems all have a negative environmental impact.

The bottom line

There are no good reasons—but many bad ones—for you to top off your tank. Just don’t do it.

 

About the Author

Natalie Josef is an automotive expert at RepairPal, the leading online source of auto repair resources and estimates. With many ASE Master certified mechanics on staff who have decades of experience, RepairPal knows all the fine points of car repair.

1 User Comment

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By , September 15, 2017
I had to replace the vapor canisters on my Camry over $400. Also replaced the canister on my Lexus over $900.