Why Does My Car Need Premium Gas?

gas-station-2022-01-unleaded-plus-premium photo by Joe Wiesenfelder

Premium gas has a higher octane rating than regular gas. An octane rating is a measurement of a gasoline’s ability to resist issues like engine-damaging knock and pinging, which can be brought on by high compression, turbocharging or supercharging typically used in high-performance engines. It can also be caused by carbon deposits in older engines.

Related: Mix Regular Gas With Premium to Save Money at the Pump

The question about using premium gas usually arises because people have seen a notation inside their car’s fuel door that mentions premium fuel. However, in terms of having to use premium fuel in newer cars, “Premium Fuel Recommended” has a different meaning than “Premium Fuel Required,” or “Premium Unleaded Fuel Only.” (Some may simply list a minimum octane rating.) If the sticker says, “Premium Fuel Recommended,” you should be able to use regular gas safely. But if it says “Required” or “Only,” you should use premium.

Higher compression in an engine produces higher horsepower, but it may require premium gas to do it. That’s because newer engines have electronic knock sensors that can detect the onset of knock and pinging and tell the computer to back off on engine ignition timing to compensate for lower-octane fuels, though power may be reduced a bit as a result. On turbocharged or supercharged engines, the boost (which makes more horsepower, but may require premium fuel to do so) can also be backed off to do the same.

For example, for the 2022 CX-5, Mazda lists its 2.5-liter turbocharged engine as producing 227 horsepower with regular fuel and 256 hp with premium 93-octane fuel. In this case, premium gas is simply recommended (some fuel-door notations might even add “for best performance”). So, if the placard inside your fuel door says, “Premium Fuel Recommended,” you should be fine using regular. You just may not get quite as much power.

What If I Accidentally Fill My Tank With Regular When Premium Is Required?

If you accidentally put regular gas in a car that requires premium, you should be OK, though there are things you should probably do to help compensate. There likely would be some premium left in the tank that would help offset the regular you put in, but you can improve your odds by driving gently and filling with premium when the tank gets down to three-quarters full — and then again at half full.

An Exception in Older Engines

Regardless of the fuel that’s recommended, drivers of older cars may notice a bit of knock and pinging coming from the engine, particularly under acceleration. This is sometimes due to carbon deposits that have built up in the cylinders, which can not only effectively raise the compression ratio, but may develop hot spots that can ignite the air-fuel mixture prematurely. In some cases, using a higher-octane fuel is enough to reduce or even get rid of the knock and ping. If that doesn’t help, it’s probably time to consult a mechanic.

The Bottom Line

If you’re not noticing a knocking or pinging noise coming from your engine, and if premium fuel is just recommended for your car, you should be able to use regular without problems. But if premium fuel is required, it’s certainly best to fill it with premium.

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