High Octane Fuel Doesn't Always Equal Better Performance or Better Gas Mileage

some engines need higher octane fuel to run properly, but the vast majority just need regular unleaded

High Octane Fuel Doesn't Always Equal Better Performance or Better Gas Mileage
Image: NCCC
March 21, 2018

Your car's manual and gas cap says to use regular unleaded. Will you get better performance and better gas mileage by switching from regular octane fuel to mid-grade or premium fuel? No. If your car was designed to run on regular unleaded gasoline, the only thing you'll notice by using a higher grade fuel is that it costs you more to fill up your tank.

What are octane ratings?

Octane ratings measure a gasoline's ability to resist engine knock, which is a rattling or pinging sound that results from premature ignition of the compressed fuel-air mixture in one or more cylinders. Most gas stations offer three octane grades: regular (usually 87 octane), mid-grade (usually 89 octane), and premium (usually 92 or 93). The ratings are posted on bright yellow stickers on each gas pump.

What happens if I don't use the right fuel for my car?

The only time you will really get a performance boost by using higher octane fuel is if your vehicle requires mid-grade or premium fuel and you use a lower octane rating than recommended. If you are using regular unleaded gasoline and your engine requires mid-grade or premium unleaded gasoline, you will likely notice a performance drop, knocking and pinging, and possibly engine damage or check engine lights. But thankfully today's cars are pretty robust and can handle the accidental fuel switch. If your vehicle doesn't require high octane fuel but you use it anyway, nothing different will happen. The biggest issues typically occur with prolonged use of improper fuel grades.

when your engine needs high octane fuel

Without getting too technical, the octane rating is determined by the compression ratio of the engine. The higher the compression ratio, the higher the octane rating and higher ignition point needed to get a proper fuel burn. If you cheap out and use a lower octane rating than recommended, you'll notice knocking and pinging from the engine, which is the fuel detonating before the proper moment. Other than the noise and possible reduced power caused by using a lower-than-recommended octane rating, you'll risk overheating your engine if the uncontrolled detonation is too great. If your vehicle requires high octane fuel, you should use the fuel grade recommended by your manufacturer. Some drivers can use a lower octane fuel with no problems, but it's no guarantee that will happen with you and your vehicle.

Octane ratings can change with altitude

If you have ever driven in high altitudes, you've noticed that the octane numbers at the pump were lower than you are used to. At those high altitudes, regular unleaded is 85 octane, not 87 octane. In those areas, if your vehicle calls for regular unleaded, choose the octane rating labeled "regular" on the pump. At high altitudes, octane values don't have to be as high in order to get the proper detonation. Don't worry if you fill up and then leave the high altitude. You'll be fine until your next fill up as fuel grades are typically ranges.

doesn't higher octane fuel clean my engine better?

High octane gasoline doesn’t outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your engine. The U.S. Environmental Protection Agency even requires that all octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against the build-up of harmful levels of engine deposits during the expected life of your car. So no matter which octane rating of fuel you use, you're getting engine cleaning additives. It is true that some mid-grade and premium brands contain higher levels of these detergents, but you probably don't need them.

what if a gas station's premium gasoline is advertised to be better?

Don't fall for clever marketing that suggest that mid-grade or premium fuel is better for your car if your manufacturer recommends regular. There's really no guarantee what you'll be getting with each fill up. If the advertisement says that the premium fuel has detergents, don't forget that regular octane fuel also has additives. You can get decent engine cleaning by using regular unleaded. If you want it a little cleaner, you can use a separate fuel injection cleaning additive periodically. If it makes you feel any better, a lot of 100,000+ mile engines that we have opened up have been extremely clean after using cheap regular unleaded fuel.

the verdict

Just use what your manufacturer recommends. It's not worth the added expense and won't boost your fuel economy or performance.