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, 2013 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". Aug 21, 2013 at 14:47
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    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, 2013 at 16:55
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    If the owner's manual doesn't say "87, except for over 5000 feet when you can use 85", I wouldn't... Sep 26, 2013 at 11:56
  • I don't think it matter that mutch. I have a turbocharged car suposed to use only 91+ but for some reason I had 87 in there for almost half a year and didn't had any problem related to that. You could get some miss fire and in this case I would personnally stick with 87
    – Rémi
    Sep 26, 2013 at 19:17

5 Answers 5


You have nothing to lose on a modern engine (except power)

Any engine with a knock sensor and computer-controlled ignition timing (e.g. modern fuel-injected designs) should be able to alter ignition timing to minimize the risk of detonation.

Difference between ignition advance at idle RPM vs 3000 RPM

When ignition timing is pulled, you should be able to feel loss of engine grunt. Here's how the sequence of events would roughly unfold in the event of a significant detonation event:

  • Detonation, characterized by untoward vibrations, are picked up by the knock sensor(s).
  • The knock sensor readings are constantly being monitored by the engine computer, which may decided to intervene and retard ("pull") the ignition timing.
  • Retarding ignition timing reduces detonation risk because it lowers the compression ratio of the engine, which lowers the effective pressure of the intake mixture:

    ▼ effective pressure → ▼ chances of detonation

The loss in engine power is the result of two effects:

  1. Engine thermodynamics

    ▼ CR → ▼ effective pressure → ▼ torque → ▼ power
  2. Pumping losses

    A greater duration of the combustion event is consumed in countering the compression stroke (upwards-traveling piston) instead of assisting the power stroke (downwards-traveling piston).


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.


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.


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.


I went to A&P school and did have a conversation very similar to this so will put in my two cents. I am not an expert in this field but I doubt those that are will want or be obliged to tell you a little history of av gas which I do believe will correlate to what you're asking. Standard av gas used to be standardized at 80 octane but was changed to 100LL octane LL standing for low lead, because someone convinced the powers that be that the higher octane would give better emissions! 100LL contains approximately 4 times the lead as the 80 octane fuel it replaced. Why did they add the lead? Anyone with a car made before the 80's can tell you, the lead is a knock inhibitor and by that I mean octane booster. When you buy fuel there is no actual Octane in it. Just an equivalent to octane in points above and below Octane. As I learned there is more energy available in a gallon of low octane fuel say 87 vs 93 where as there may be more power available with the 93 at the expense of mpg and more mpg per lower octane! Higher octane is a scam for those seeking better mpg, for those wanting more power you're car has to be tuned for it and bigger injectors to spray more fuel. When gas sits for a prolonged amount of time it loses some of it's octane rating, so if you started with 87 octane and let it sit a month then start it up probability is that the fuel you are burning is below 85 Octane rating so anybody want to be the judge of will a car run properly below 87 Octane?

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    The lead was also used as a means to lubricate the valve and valve seats within the head. It's the reason they had to go to hardened valve seats in newer engines; to be able to stand up to the abuse which was folded onto them when the EPA took the lead out. Also, do you have any references for anything about gas losing octane over time? Never heard of that one before. Aug 7, 2016 at 20:25
  • This answer is incorrect. Higher-octane fuels can have higher energy content (see e.g. appropedia.org/Energy_content_of_fuels). Higher-octane fuels are pointless in an engine designed for low-octane fuel, but engines designed for high-octane fuels will achieve better performance when run with high-octane fuels. When you let gasoline sit, the components with the lowest boiling points will evaporate first, IMO this is more likely to leave you with a high-octane fuel. Petrol does contain actual octane, in a mix with loads of other compounds.
    – Hobbes
    Dec 12, 2016 at 13:08

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