Why Topping Off Your Gas Tank Is a Bad Idea

When you visit the gas station to fill up your gas tank, what do you do when the pump clicks off to signal that the tank is full? If you try to squeeze in a few more drops, you’re not doing yourself any favors.

People top off their tanks for any number of reasons — trying to round up to the next dollar or gallon, trying to go longer between fill-ups, or just habit. But doing so can hurt the environment, your car and your wallet.

Gasoline’s problematic properties

Putting as much gas as possible in your car sounds like a good idea. But there are two primary reasons why it can cause trouble.

• Gasoline gives off vapors: This is one reason why gas burns so well in your car’s engine. But these vapors can be a big problem if they escape when you fill your tank.
• Gasoline expands when it gets warm: The underground storage tanks where gas stations keep their supplies stay very cool. But if the temperature aboveground is warmer than the storage tanks, gasoline can expand. This is most likely to happen after you pump the gas into your car’s tank. 

Gas vapors are a major component of vehicle-generated air pollution. When vapors escape during the fueling process, they react with sunlight to produce smog. 

In an attempt to minimize the harmful effects of these evaporative emissions, many gas stations have vapor recovery systems. These systems use fueling nozzles that have rubber seals, to prevent the fuel vapors from escaping when you fill up. 

Your car’s gas tank also has a vapor recovery system. This system, which uses a charcoal canister, is designed to collect any excess vapors in your tank. The vapors are then returned to the engine to be burned. 

3 reasons why topping off is bad

1. It can hurt your car: When you fill up your gas tank to the tippy-top, you may not be leaving enough room for the vapor recovery system in your car to do its job. If the fuel level inside the tank is too high, causing the charcoal canister to suck in liquid fuel instead of vapors, it can be damaged and need replacement — and that can be expensive. 

2. It can hurt the environment: If you overfill your tank, you can spill gasoline on the ground. Spilled gas will instantly evaporate, go into the atmosphere, and create smog when the sunlight hits it. Gasoline is also toxic, and you definitely don’t want to breathe the fumes, or get it on your skin or clothes. Plus, it’s a fire hazard.

3. It can hurt your wallet: The vapor recovery systems at gas stations are designed to recapture not only gasoline vapors, but also any excess gasoline being pumped into a car’s tank. This means that the extra gasoline may actually be going right back into the gas station’s storage tanks — but you’re still getting charged for it.