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  1. Default Driving with Octane 85 over 6000 feet altitude

    First time in Utah went to a gas station and just choose the lowest octane rating fuel I saw... was obviously a 85 and I first panicked till I read on the internet that the worst that can happen with newer cars is lower performance or worse mpg. As I'm currently driving on the I-80 E from Salt Lake City I just wanna share my experiences with it...
    So far I didn't had any problems with it, neither performance issue nor MPG which is still at 41.4. It's good to know that after Evanston I was never lower than 6000 feet again, I don't know how the engine would handle it when I would drive around in lower altitude.
    For now I'm happy that the engine runs smoothly and I'm definitely gonna pay attention and get 87 again.

    PS: I'm driving a Nissan Altima

  2. #2
    Join Date
    Oct 2008
    Location
    Joplin MO
    Posts
    9,536

    Default

    You will have no problem whatsoever with 85 octane in any vehicle rated for 87, even at lower elevations. I've run it down as low as 2000 feet. No need to run any higher unless higher than 87 is specified or your engine is turbocharged.

  3. #3
    Join Date
    Mar 2005
    Location
    Tucson, AZ
    Posts
    9,578

    Default What 'Octane' Means

    The octane rating of a fuel is simply a measure (I'll spare you the boring chemistry details) of it's ability to burn in an engine without causing the engine to 'knock' due to pre-ignition. It's mostly relevant to high-compression engines (Think Chrysler's hemi-driven muscle cars of the '70s) and is far less relevant in today's fuel-injected, low-compression, computer-controlled, high-mileage cars. Indeed, e85 graded fuel (85 octane fuel with ethanol added), is common and sometimes mandated in some states. You can run your car safely on any octane fuel that doesn't cause knocking under normal usage. Using higher octane, and higher cost, fuel is just a waste. Should your car develop knocking at high altitude and under load conditions, simply fill up with a higher octane fuel for as long as you are operating under such conditions and then switch back to lower rated fuels when you return to normal driving conditions.

    AZBuck

  4. #4
    Join Date
    May 2003
    Location
    Green County, Wisconsin
    Posts
    13,396

    Default clarification

    Quote Originally Posted by AZBuck View Post
    Indeed, e85 graded fuel (85 octane fuel with ethanol added), is common and sometimes mandated in some states.
    Just a word of caution, as there can be some confusing terminology here. E85 more commonly refers to a gas-ethanol blended fuel that is 85% Ethanol. Such a fuel (which has an octane rating of around 100) is not recommended for most cars, unless it's labeled as a Flex Fuel vehicle - such vehicles usually also have a yellow gas cap.

    What Buck is talking about, is generally labeled "regular unleaded" across most of the country and is actually 85 octane gasoline mixed with 10% ethanol (e10), which results in an 87 octane rating.

    Now getting into the specifics of high elevation, it is quite common to see 85 or 86 octane in the mountain states. As Buck mentioned, all an octane rating really does is measure the point at which the fuel will ignite due to compression. In high elevation area with reduced oxygen, it takes more compression to ignite the same fuel, so less octane is needed to get the same results.

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