Stopping at a gas station to refuel your car is inconvenient—especially when you have to wait for a pump, or drive out of your way to find somewhere to fill up. So, in order to get the most out of a tank of gas, you may opt to top it off after the pump stopped.
But according to experts at Consumer Reports and the American Automobile Association (AAA), squeezing a few extra drops into your tank is a bad idea. Here’s what to know.
Why you shouldn’t top off your gas tank
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Even if you’ve been topping off your gas tank for decades without a problem, automotive experts have identified three reasons why you might want to break that habit:
It could damage your car
“There’s a lot more going on in a car’s fuel tank than just a tank,” says John Ibbotson, Consumer Reports’ chief mechanic. “Fuel systems have become more sophisticated over time to keep up with increasingly stricter emission laws.”
More specifically, Ibbotson is referring to a vehicle’s onboard refueling vapor recovery (ORVR) system, which consists of a charcoal-lined canister that collects fuel vapor as a car is being refueled, then absorbs it through activated carbon. Not only does this help to reduce hydrocarbon emissions by approximately 95 percent, but it also helps trap these fumes to burn as useable fuel.
ORVR systems have been around since the 1970s, but in the 1990s, the Environmental Protection Agency (EPA) made them mandatory in practically all newly manufactured vehicles, phasing in these requirements between 1998 and 2006.
When a gas pump shuts off on its own, but you top off your tank anyway, the extra fuel can saturate the charcoal canister, causing your “check engine” light to turn on, and potentially damaging the system. According to Ibbotson, that completely avoidable repair could end up costing hundreds of dollars.
In addition, when the ORVR system is full of fuel, your car may emit more hazardous fumes than usual, and it can cause a reduction in your gas mileage, per AAA.
You’re losing money
There are also regulations in place requiring the pumps at gas stations to have their own vapor recovery systems in order to prevent the fumes from polluting the air, and excess gas from spilling onto the ground or customer. According to experts at AAA, this is what causes the pump to turn off when your tank is full.
When this happens, and you decide to top off the tank, that extra fuel is drawn back into the pump. In other words, you’re paying for gas that you’re not able to use.
It can harm the environment
If you try topping off your tank and end up spilling the flammable liquid on the ground, it can contaminate the groundwater and surface water, the soil, and the air, when it evaporates.