During our recent trip to Montana and Wyoming, I was surprised to find octane 85 and 85.5 gas at the gas stations. I had the false idea that octane ratings had to do with fuel efficiency, fuel “quality”, and or pollutants. I had some homework to do.
What is Octane?
The octane rating of gasoline measures its resistance to spontaneous ignition under compression in internal combustion engines. A higher octane level does not mean a higher energy content in the fuel. In theory, you should be able to go the same distance on any grade of gas. Rather, the higher the octane rating, the higher the activation energy required to ignite it. This is why high performance engines that have high-compression cylinders require higher octane levels. Lower octane grade gas would combust prematurely and cause engine “knock.”
There’s some other chemistry involved in calculating the rating, but I don’t care to research at the moment.
Why 85, When The Lowest I’ve Seen Elsewhere is 87?
In the mountains, the thin air does funny things. As engines take in less oxygen per cycle, they produce less compression than they would at sea level. Because of the changes in atmospheric pressure, 85 octane at high elevation is the performance equivalent of 87 octane at low elevation. If you drive from the mountains down to sea level on one tank of gas though, you might experience some knocking.
So no, for normal cars at high elevation, you might as well save some money and go with the 85 octane gas, even when your owner’s manual recommends 87. From what I could find, it won’t hurt your engine performance, fuel efficiency, or tailpipe emissions.
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