Is It Bad to Top Off Your Gas Tank?

Quick summary

• Topping off means continuing to pump gas after the nozzle automatically clicks off.
• Overfilling can push liquid fuel into vapor recovery components where it does not belong.
• Damage to the EVAP system or charcoal canister can trigger a check engine light.
• Spilled fuel and excess vapors can create safety risks and harm the environment.
• Extra fuel added after shutoff often does not enter the tank and can waste money.

While it may seem like a good idea to cram as much precious fuel into your gas tank as it can possibly hold—especially as gas prices rise—topping off can wreak havoc on your car, the environment, and your finances.

What topping off means at the pump

So, what is topping off gas? It’s when you fill your tank up to your vehicle’s recommended level—indicated by the gas pump automatically clicking off—and then squeeze some more gas into the tank, filling it to the brim. It may seem like gas pumps click off prematurely and that your vehicle can take another gallon or two. Or, maybe premium gas is extra cheap this week, so you really want to get that dollar amount displayed on the pump to a nice round number. The reality is that automatic shutoffs on gas pumps exist for many good reasons.

The main reason is to protect you, the consumer, as well as the gas station and the environment from the hazards caused by spilled gasoline and escaped vapors. Not only is gasoline a hazardous material due to its incredibly flammable nature, but gas evaporates quickly, producing harmful vapors that contribute to poor air quality. When spilled on the ground as consumers try to squeeze that last little bit of gas into their tanks, it creates toxic sewage runoff.

Because of these safety and environmental concerns, gas stations are required per government regulations to equip their pumps with vapor recovery systems that prevent vapors from escaping and fuel from spilling on the ground. This means that any gas you pump after your pump clicks off can wind up being drawn back into the station’s storage tanks, meaning you’re paying for gas that’s not going into your vehicle. Yes, that’s right: You may end up paying for gas that’s not going into your tank but, instead, into the gas station’s tanks.

Why topping off can damage your car

By topping off your gas tank even once, you begin to run the risk of having liquid fuel slosh and flow into the delicate charcoal filters in these vapor recovery systems. Once they become saturated in liquid fuel, their capacity to capture and burn vapors successfully can be reduced or ruined completely. Not only do saturated filters in the EVAP and ORVR systems make your vehicle emit more hazardous emissions, but they can also impact engine performance and reduce fuel economy. Eventually, these saturated filters may even lead to a “check engine” light or costly repair that was completely avoidable.

While it may be tempting and you may feel like you’re rewarding yourself with fewer visits to the gas station, there are far better ways to save on gas and become a safer driver in the process. No matter which way you slice it, topping off your gas tank is a bad idea for all the following reasons.

• Spilled gasoline and escaped vapors are environmentally harmful.
• Overfilling your gas tank saturates your vehicle’s vapor recovery system, possibly leading to permanent vehicle damage and preventable repairs.
• Gas stations are equipped with vapor recovery systems that recover gas pumped after the automatic shutoff. So, remember, topping off means you’re paying for gas that’s going back into the station’s tanks.

Why topping off is bad for safety and the environment

Gasoline is highly flammable and evaporates quickly when exposed to heat. Because of this volatility, not only are spilled gasoline vapors bad for the environment, but highly flammable fumes can be incredibly dangerous. This is why, in addition to the vapor recovery systems equipped on gas pumps, there are several on-board systems inside every vehicle that are designed to minimize and process the gas vapors present in your tank as well as those that are generated during the fueling process. 

The Onboard Refueling Vapor Recovery (ORVR) and Evaporative Emission Control (EVAP) systems are required on each new vehicle sold in the US. While each system consists of a charcoal-lined canister that collects fuel vapor and absorbs it with activated carbon, the ORVR is responsible for capturing vapor as the gas pumps into your vehicle, effectively reducing hydrocarbon emissions during refueling by about 95 percent. The EVAP, on the other hand, manages gas vapors inside the fuel tank. All cars sold since the late 1990s are equipped with both of these closed-circuit systems that help prevent polluting vapors from escaping and trap and burn fumes as usable fuel.

What to do instead of topping off

Simply stop at the first click. The pump is designed to shut off when your gas tank is properly full. Remember to replace the nozzle and cap immediately to avoid vapor leaks or spills and/or driving away from the gas station with an open fuel door.

What if you already overfilled once?

While habitually overfilling your gas tank isn’t wise, doing it accidentally once or even occasionally shouldn’t cause any noticeable damage to your vehicle. In the event you overfill your tank, the most important thing is to try to clean any spilled gas off yourself and the side of your vehicle. Gas is an irritant that can harm your skin and damage the finish and color of your car’s paint over time. 

Is it safe to drive after overfilling your gas tank?

While regularly overfilling your gas tank can lead to delicate vehicle components being soaked in fuel, there’s nothing inherently dangerous about driving with an overfilled tank. If you’ve accidentally overfilled, you won’t even notice a difference in how your vehicle drives other than getting just a few extra miles on the tank. Over time and after repeatedly topping off your tank, however, you may notice your vehicle experiences difficulty starting, lingering gas odors, and/or a reduction in fuel efficiency, just to name a few negative effects.