While it’s tempting to squeeze in as much gas as possible when you fill your tank, filling it past the pump’s first automatic shut-off can damage your car to the tune of hundreds of dollars. It can also adversely affect the environment and even risk your safety.

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What can get damaged by overfilling your tank is the vehicle’s evaporative emissions control system. These systems have been in use for decades to control harmful emissions that occur when gasoline vaporizes, which it does quite readily. In any concentration, these vapors can be harmful to both the environment and anyone breathing them in.

Prior to having these systems, any gas vapors would collect at the top of the tank and fuel-filler neck, escaping either while you were driving (through the gas cap’s vent hole) or when you opened the gas cap to fill the tank. What the control system does is trap these vapors, then burn them in the engine along with the liquid gas.

Typically, the way this system works is by using a vacuum created by the engine to draw these vapors into a large canister filled with charcoal and, when the time is right, draw them into the engine to be burned.

These systems are only designed to work with vapors, not liquids. If you overfill your tank, it can cover the vapor intake hole with liquid gas, which can then be sucked into the charcoal canister. This can damage the canister and possibly other parts of the system, which will cause the car’s check-engine light to come on and could potentially cost hundreds of dollars to repair.

Adding to the concern of overfilling is that gasoline expands in volume by about 1% for every 15 degrees Fahrenheit, according to Consumer Reports. As it’s not uncommon for a day’s temperature range to vary by 30 degrees, that means that 15 gallons of gas put into a tank when it’s cool will expand by more than a quart when it warms up by 30 degrees. If the tank is overfilled, that could be enough to push gas into the charcoal canister’s feed line or force a leak, which is a major safety issue.

Because preventing gasoline vapors from escaping into the atmosphere is so important, a gas station’s fuel pumps are usually equipped with vapor recovery systems of their own to trap vapors generated as you’re filling your car with gas. That’s the reason for the large rubber collar or accordion-like tube on the pump handle that covers the opening of your filler neck while you’re filling your tank.

If you fill your tank past the first click, this system can suck in gas you just put in — and paid for — and could also damage the pump’s vapor control system. Overfilling can also cause gas to splash out, pouring a little more of your money onto the ground. This, however, is pocket change compared to the cost of the damage it could do to your car.

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