85 vs. 91 Octane (4350ft elevation)

Discussion in 'Fuel Economy' started by MPG, Jul 28, 2008.

  1. MPG

    MPG Banned

    Hello all, I filled up today with 91 octane fuel. I have heard some people say you will get better fe, and some say you get worse fe.
    I know that if you car does not require premium fuel and you run it you will not gain any performance, and all your doing is wasting money.

    But the question is which octane fuel will give you the best fe?

    Thanks, MPG
  2. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    85 would be the correct answer if your car doesn't require premium.

    At your elevation (a little lower than me) your effective cylinder pressure is much lower than sea level and therefore we can get away with 85 vs the lower altitude 87.
  3. RoadTrain

    RoadTrain Member

    Your car can't perform better in any way with higher octane unless the ECU is tuned for it. A higher octane will allow for more advanced timing, but if the ECU doesn't dial that in, it is a waste of $$
  4. some_other_dave

    some_other_dave Well-Known Member

    Many cars these days require no lower than XX octane (varies depending on the car), but will actually give you better performance if you use a higher grade of fuel. Some of those will give you better fuel economy if you use the higher-grade stuff, but some will not. There doesn't seem to be a lot of rhyme or reason to which do and which do not, sad to say.

    A very few of the people who report improvements report that the improvements are actually worthwhile, as they get a higher gain than the extra cost of the fuel. Most who report improvements seem to gain less than the extra cost of high-grade fuel.

    If you have instrumentation on your car (like an SG II) and a good repeatable test set, I'd say try each and see what the results are. Your car just might like the 91, or it may not make any difference. You might even see a slight decrease in FE...


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