Have you ever filled your car with fuel only to realize you accidentally hit “regular” instead “premium” or vice versa? If this has happened to you, there’s no need to panic. You probably haven’t destroyed your engine, and it probably won’t do any harm. But it’s not a mistake you should make very often.
How to find the right octane gas for a car
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The easiest way to determine the octane recommended or required for your car’s fuel is to check the owner’s manual. If you don’t have the manual handy, you can Google your car’s make, model, and year and get the info that way. You should, of course, use the octane gas that is recommended or required.
But what if you use another octane by mistake? The three tiers of gasoline (87, 89, and 93 octane) are all next to each other on the pump, and it’s not hard to absentmindedly push the wrong button.
What happens if you use a higher octane gas than is recommended?
If you opt for a higher octane than your manual recommends—say, 89 instead of 87—your car will be perfectly fine. It will not, however, improve your car’s mileage or give it any sort of extra performance boost. The only thing it will do is cost you extra money, so don’t do this.
What happens if you use a lower octane gas than your car requires?
For some car models, higher octane fuel is recommended instead of required. If that’s you, the lower stuff should be sufficient enough to not cause problems. But if you hear knocking and pinging noises, or you’re not getting the mileage you should, switch to higher octane gas at your next fill up.
If you put lower octane gas in a car that requires premium, everything will still probably be fine, but you should be concerned about the “probably” part. In older cars, putting a lower octane gas in the tank might cause knocking and pinging and will result in reduced performance. In most cases, it won’t damage your engine, but it could, depending on the engine, the car, how you drive it, and other factors.
Newer cars handle lower octane fuel better than older ones. Onboard computers can smooth things out to an extent—sensors in modern engines have the capacity to adjust the ignition system to accommodate lower-octane gasoline. But in many cars, this only works in “steady state conditions,” like cruising down the road, or driving steadily up a hill, so this is not license to ignore the octane requirements.
In a worst-case scenario, the premature ignition of fuel caused by using the wrong octane gasoline can damage your engine’s pistons, even potentially voiding your warranty, especially if you make a habit out of it.
In a best-case scenario, your car will be fine, but your engine will run less efficiently and the amount of mileage you get per gallon will suffer. This is why it will not actually save you money to put “cheap” gas in a car that requires a more expensive option.
What you should do if you use the wrong octane
Don’t freak out, but keep an ear out for pinging or knocking sounds, which could be a sign that the gas and air mixture in your car’s cylinders isn’t burning exactly as intended. This pinging or knocking will make your engine less efficient and may cause damage over a long period of time—but one slip up is probably fine.
This story was originally published in August 2016 and has since been updated to provide more thorough and current information and adhere to Lifehacker style.