What Is 85-Octane Gas and Why Isn't It Available Everywhere?

Carbureted engines once benefited from lower octane fuel at high altitude.


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Most fuel pumps across the United States offer three tiers of gasoline, ranked by their octane number: 87 octane, 88-89 octane, and 91-94. Octane numbers describe gasoline's ability to resist knock, or pre-ignition, which occurs when compression of the air and fuel mixture in a cylinder causes ignition before the spark plug fires. The higher the octane number, the more knock-resistant the fuel, making high-octane fuel better suited for high-compression, high-performance engines. The higher the octane also means higher cost per gallon.

Occasionally, you might encounter a fourth octane number at the fuel pump. You won't find 85-octane fuel in every state — and in some places you’ll find 86-octane gas — but there are some regions where it can be found at nearly every gas station. Here's why 85-octane fuel exists and whether you should put it in your car.

What Is 85-Octane Gas?

Modern vehicles feature engines tuned to run on a specific grade of gasoline. A motor designed to run on 87 octane won't see any performance benefits when using a higher-octane fuel. On the other hand, a car intended to run on 91 octane will typically experience performance issues if you use a lower-octane fuel, as the engine's computer controller attempts to compensate for the lower knock resistance.

These engine tunes are programmed assuming the vehicle is driving at or near sea level. At higher altitudes, however, lower atmospheric pressures change the reaction inside each cylinder. Specifically, less oxygen is available the higher you get above sea level, requiring less fuel to mix and burn when producing power. For older vehicles with carburetors, this reduced the knock-resistance requirement below the 87-octane standard, allowing an 85-octane fuel to perform like an 87-octane fuel.

Where Is 85-Octane Gas Found?

Fuel with 85 octane is most easily found in parts of the U.S. where high-elevation driving is common, including Colorado, Utah, Wyoming, Idaho, Montana, and New Mexico. In these states, many towns sit thousands of feet above sea level, and 85 octane has carved out a foothold as the base grade of fuel during the era of carburetion. It also offered savings for those who live at altitude.

Is 85-Octane Gas Safe for My Car?

The correct fuel to use in your vehicle is listed in its owner's manual and on the sticker inside the fuel filler door. The advent of modern fuel-injection systems and sophisticated computer controls means that 85-octane fuel is no longer a cost-effective choice for modern automobiles. Current cars and trucks are less tolerant of low-octane fuel than their predecessors, regardless of altitude. Potential problems from using 85-octane fuel (such as pinging, power loss, reduced fuel economy, and voiding of the warranty) outweigh the savings from the fuel's lower price.

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Benjamin Hunting
Benjamin Hunting is a writer and podcast host who contributes to a number of newspapers, automotive magazines, and online publications. More than a decade into his career, he enjoys keeping the shiny side up during track days and always has one too many classic vehicle projects partially disassembled in his garage at any given time. Remember, if it's not leaking, it's probably empty.