chemistry

All About Octane

Gas pumps offer lots of choices: Regular, mid-grade, premium, 85, 87, 91, 93… the array can be dizzying. Which gasoline should you put in your car?


By Aaron Gold

The Short Answer

Use the least-expensive (lowest octane) gasoline required for your car (except E85 and diesel — see below), as listed in your owner’s handbook.

The Long Answer

The numbers (87, 89, 91 etc.) refer to the octane rating, a measure of a gasoline’s resistance to detonation. Your engine uses a spark to ignite a mixture of gasoline and air, which is supposed to combust in an orderly fashion. Sometimes the heat and compression can cause pockets of fuel to explode before they are supposed to; that’s detonation (also known as knocking for the sound that it makes). The higher the octane, the more resistant the fuel is to detonation.

Most engines are designed to run on regular (87 octane) gasoline, but some cars use high-compression engines, which squeeze the air-fuel mixture down to a smaller size before igniting it with a spark. These engines are more prone to detonation, so they often require premium fuel to work most efficiently — 91 or 93 octane.

Premium fuel: Required VS. Recommended

Your owner’s manual will tell you the minimum acceptable octane for your car, and this information may also be written on the gas cap or the fuel filler door. If your car calls for anything higher than 87 octane, note if it is recommended or required. Recommended means that premium fuel will give you better performance (and often better gas mileage), but the car will run fine on lower octane. If it says required, you should always buy the higher octane fuel to avoid engine damage.

Note: Some European cars list two sets of octane numbers, one labeled as RON and the other as AKI or (R+M)/2. American gas stations use the AKI and (R+M/2) measurements. If you’re not 100% sure which octane your car needs, ask your dealer or call your car manufacturer’s customer service line.

What Happens If I Use Regular In A Car That Requires Premium?

In the past, running 87 octane fuel in a car that required premium would cause the engine to “knock” and could lead to severe engine damage. Today’s cars have knock sensors that can detect detonation and recalibrate the engine to compensate, although the engine will produce less power and may get worse gas mileage. If your car requires premium fuel, using gasoline with too low an octane rating can potentially cause damage and may void your warranty.

What Happens If I Use Premium In A Car That Requires Regular?

Nothing, except you spend more money. Some people believe that high-octane gas will improve performance, but it doesn’t — remember, octane refers to the detonation resistance of the gasoline, not the amount of power it contains. Some oil companies advertise that their high-octane fuel does a better job of keeping engines clean, but the truth is that the Environmental Protection Agency requires cleaning additives in all grades of gasoline.

What About Mid-Grade?

Some stations offer mid-grade fuel (usually 89 octane). There are a handful of cars that require 89 octane gas, but most cars are designed for either regular or premium. If your car recommends (but does not require) premium, mid-grade will give you some of the performance benefits, but you will still see reduced power and fuel economy. If your car requires regular, mid-grade will not improve the way it runs.

What If The right Octane Isn’t Available?

In some high-altitude areas, you’ll find 86 or even 85 octane gasoline sold as “regular” grade. At higher altitudes, the thinner air compensates for the gasoline’s lower octane, so the engine can usually run without detonation. If you are staying in the area, you can generally use the same level of gasoline you would at lower altitudes, i.e. regular, mid-grade or premium. If you will be traveling out of the high-altitude area, go by the numbers and use the same octane you would at lower altitudes. Note that most automaker suggests sticking to the recommended octane rating, even at high altitude.

What About E85 And Diesel?

E85 is a blend of 15% gasoline and 85% ethanol, a plant-based gasoline substitute. E85 should only be run in “Flex Fuel” vehicles that are designed to use it. In the United States, E85-capable cars will have a yellow gas cap that says E85. Anon-flex-fuel car may be able to run on E85, but it can damage the fuel system and cause leaks.

Diesel fuel is intended specifically for cars that have diesel engines. Gasoline engines will not run on diesel fuel (and diesel engines will not run on gasoline or E85). Diesel pumps in the United States have a large-diameter nozzle that will not fit into the fuel filler neck of a gasoline-powered car. If you drive a diesel car, you should only use diesel fuel — gasoline or E85 will cause expensive damage.

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