When you pull into a gas station to refuel your vehicle, you'll likely notice a warning or two about how you shouldn't top off the tank. Plenty of drivers will still add some extra gasoline after the automatic shutoff kicks in, perhaps not realizing that doing so can be a waste of money or even harm their vehicle.

The temptation to top off often occurs when the pump deems your tank full and the amount you owe is an awkward number. Because they want a simpler job of balancing their checkbook, or maybe because of our natural preference for nice round numbers, a driver adds some extra fuel to reach the desired fee. Others may not think that the pump provided an adequate amount of gas before the shutoff.

The pump includes a mechanism to allow air to be displaced from a fuel tank, and it will allow gasoline to be added to the tank for as long as air can be moved out of it. Once the liquid reaches a level where the flow of air is displaced, the automatic shutoff is displaced. In general, the system works well enough that your fuel gauge should register as full if you pump gas until the shutoff kicks in.

The most obvious risk of trying to add gasoline beyond this point is that gasoline will spill out of the tank. The Santa Barbara Air Pollution Control District in California says gasoline contains harmful materials such as benzene and toluene, which can increase smog levels or affect the health of nearby people when they are released into the atmosphere. The organization says spilling an ounce of gasoline will release the same amount of volatile organic compounds as you would produce by driving a vehicle for 56 miles.

Filling up too much of your gas tank can also inhibit the vehicle's performance. The Environmental Protection Agency says some extra room is needed in the tank for vapors to expand. Louetta Automotive, a business based in Cypress, Texas, says older cars had holes in the gas cap to allow this vapor to escape, but this practice was abandoned once it was discovered that these vapors contributed to health and environmental problems. Modern vehicles now have a vapor recovery system to recapture this fuel and make the engine run more efficiently.

When you top off the tank, gas may spill or evaporate into the vapor collection system. Tom Moor, writing for Angie's List, says liquid fuel entering the system can damage the charcoal canister used to filter gaseous vapors. The excess gas can also increase the chance of a leak in the system through increased pressure, make your vehicle run poorly, and release more emissions into the atmosphere.

If you try to add extra gasoline after the pump shuts off, you may end up paying for fuel you're not actually getting. The EPA says pumps have vapor recovery systems to feed excess fuel and vapors back into the station's tank to help prevent pollution; when you top off, it's more likely that this process will occur and the fuel won't end up in your tank.

Just as topping off can damage your vehicle's vapor recovery system, it can damage this system on the pump. The Santa Barbara Air Pollution Control District says fuel may also get stuck in the hose, creating the potential for a spill when the next driver uses the pump. The effectiveness of the vapor recovery system might be reduced, increasing the possibility of air pollution or a malfunctioning pump.

If you think the automatic shutoff has kicked in too early, inform a station attendant instead of continuing to add fuel. You should also let an attendant know if you notice any other issues that may have been caused by another driver topping off, such as a strong smell of gasoline or fuel spilling from the nozzle when you pick it up.