Have you ever worried about old gas in your car’s tank? Can old gas impact your car’s performance? Find out the effects of old gas in your car’s tank.
Do You Need to Worry About Old Gas in Your Car’s Tank?
Updated on Apr. 04, 2025
If you have a car that hasn’t been driven for a while, you might be wondering if the gas in the tank is still OK or if it needs to be removed and fresh gas added. While gas that has been sitting does eventually go bad, it can still be cleaned and treated using a fuel stabilizer or even by topping off your tank with new gas. Read below to see what happens when gas gets old and what you can do to ensure your engine is running efficiently.
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Is old gas in the tank bad for your car? The quick answer
In almost every case, old gas is not an issue. Gas that sits does slowly go bad. However, 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 — how explosive it is — 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, gas that is more than two month old is generally OK to use with only minor decreases in performance. Gas that is older than a year can cause issues, like engine knocking, sputtering and clogged injectors. 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?
Ronald Montoya, senior consumer advice editor at Edmunds, suggests topping off the engine to prevent water 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.
FAQs
Can old gas be cleaned or treated?
Fuel stabilizers are an effective way to treat old gas if your car has been sitting around for a while. They help to clean the fuel system and improve the quality of the gasoline, which helps engine performance.
How can I tell if my gas is bad?
Rough idling, difficulty starting, sputtering and a check engine light could all point to signs of gas that has gone bad if your car has been sitting around for a while.
About the Experts
Ronald Montoya is a senior consumer advice editor at Edmunds.
John Ibbotson  is the chief mechanic at Consumer Reports.
