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Here in Colorado, gas stations sell "regular" gasoline which is rated at 85 octane ((M+R)/2). In most of the rest of the US, 87 octane is the lowest grade available. (As noted previously, higher octane fuel can withstand higher compression without detonation.)

According to this article from AAA, the stated reason is that most of Colorado is at high elevation, above 5000 feet (about 1500 meters), and the thinner air tends to prevent detonation. Therefore, in theory, one can use the cheaper, lower octane fuel without problems, and save some money. (85 octane currently tends to be about 3-5% cheaper than 87, which is also sold here.) However, the article also says that a 2001 state legislative report called this theory into question, with respect to vehicles newer than 1984. I wasn't able to find a copy of this report to see the specifics; in any case, engines have advanced considerably since 2001, so it isn't clear whether that research would still apply. Is there more recent research on this topic?

The owner's manual for my car (a 2006 Honda Civic) specifies 87 octane fuel. However, the manual presumably was written for the majority of the customers who live near sea level, and may not have considered altitude effects. So I would like to know whether I can safely use 85 octane fuel without risking problems.

As a follow-up, my understanding is that modern engines detect detonation and adjust to eliminate it, at the cost of some performance. Thus, if I do switch to 85 octane and it turns out to be insufficient for my engine, how could I tell?

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If you read the rest of the article you see "The Colorado Legislative Council contradicted that research in a 2001 study, which showed that the altitude difference might apply only to older cars (pre-1984)." In a modern NA car, the owner's manual wins. In a turbo car, the altitude matters much less: our engines go and grab as much oxygen as they can get! – Bob Cross Aug 21 '13 at 12:41
Yes, as I mentioned, I saw the reference to that study. But I'm not willing to rely on it without reading it (as far as I can tell it isn't readily accessible) and in any case it would be 12 years old. And they only said "might". – Nate Eldredge Aug 21 '13 at 14:47
The original finding was from the American Petroleum Institute, not an objective source. Our guidance on the site is to rely on your owner's manual - see the related questions on the right for more information. – Bob Cross Aug 21 '13 at 16:55
I'm with Bob. Use what the manufacturer says to and let your powertrain controller sort it out. It will adjust the timing to eliminate knock. You might get by with 85 instead of 87 at the cost of retarded timing, but I am not an engineer and I did not design your vehicle. If a service department ever asks you if you put anything less than 87 in it before performing warranty service, I think you want to be able to say "No". – Mark Johnson Aug 22 '13 at 23:56
If the owner's manual doesn't say "87, except for over 5000 feet when you can use 85", I wouldn't... – Brian Knoblauch Sep 26 '13 at 11:56

Most likely put in whatever the manual is saying, else if you are going to service, you might have some trouble over there. Also it is for the sake of durability of engine internals.

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You are of course correct. Higher altitudes require lower octane. A quick way of getting your car to play nice is to disconnect the battery for 30 minutes and reconnect. this will reset your fuel trims (unfortunately also your radio, clock, etc). It will help with getting your car to play nice with the new grade of fuel and thinner air.

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Very few cars have active octane analysis whereby the timing is adjusted on the fly with fuel grade. VERY few and restricted to more expensive and/or performance oriented luxury vehicles. Else, all that will happen is if your engine starts knocking, it retards and you lose a massive amount of power, gas mileage suffers as well.

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