Exactly how much impact can old gas in your tank have on your car’s performance?

Is old gas in the tank bad for your car?
In almost every case, old gas is not an issue. Gas that sits, even for a few months can be redeemed by topping off the tank with fresh gas. When the fresh gas mixes with the older gas, the motor will operate properly. John Ibbotson, chief mechanic at Consumer Reports, says that, “The new gas will mix with what’s already in your tank, and any variance in the octane will be adjusted for automatically by your car’s engine computer.” The adjustment will get the engine running back to normal.

What happens when gas gets old?
If gas sits, it begins to degrade in a couple of ways. In time, gas will lose octane. Octane is the combustible part of gasoline. The higher the octane rating (think 87, 89, 93), the better the air-fuel mixture and the better the combustion in the cylinders.

Old gas also loses its volatility as it ages. The decrease in volatility decreases engine performance. Residues and water from gas burning can also build up in the engine and gas as it sits. None of these bode well for engine function.

How old is too old for gas?
Degradation occurs from the get-go but most gas stays fresh for a month or two without issue. However, a gas that is more than two months old is generally okay to use with only minor decreases in performance. The gas that is older than a year can cause issues, like engine knocking, sputtering, and clogged injectors. (Here are 15 car problems you can diagnose and fix yourself.) Bad gas can be drained from the tank to prevent damage to the engine. One caveat to keep in mind is that there is no way of knowing how old the gas is when you first pump it into the car.

What can you do to protect the car if you store it?
You may top off the engine to prevent water from accumulating in the engine and adding a fuel stabilizer in the engine to slow the breakdown of the gas. Fuel stabilizers can extend the life of gas for up to 15 months. Be sure to add the stabilizer to fresh gas. Finally, if you can’t store the car inside cover it with an all-weather cover.