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Thread: Manually calculating gear ratio inputs

  1. #1
    Join Date
    Jan 2015
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    0

    Default Manually calculating gear ratio inputs

    Hi,

    I have a G2X (red). Although I can monitor my speed via GPS, I am interested in setting
    up the gear ratios in order to see which gear I'm in. I know I can do it "automagically"
    in Setup2, but would rather do it the manual way.

    Have done a search, but can't find a "method" to to calculate them properly.

    I have a 944S and my current gear ratios are:

    1 - 3.500:1
    2 - 2.059:1
    3 - 1.400:1
    4 - 1.034:1
    5 - 0.829:1

    I'm running 235/50/15 tires which gives me a circumference of 73 inches (if I measured correctly)

    This might be a really basic question.....but.....How do I calculate the ratios for proper entry? Or do
    I simply enter them as they are?

    Thanks,

    Jim

  2. #2

    Default Re: Manually calculating gear ratio inputs

    I get 76 in for your tire diameter (=pi()*(235*0.5*2/25.4+15)). If you actually measured 73 in, then use that. The best number to use is the revs/mi number that the tire manufacturer provides. If you don't know this number, see if Tire Rack has this number for your tires. If you still can't gets revs/mi, then you can calculate it.

    revs/mi = 5280*12/Tire Circumference

    Once you have the revs/mi number for the tire, you can calculate the required ratio as follows.

    ratio = GearX*GearFinal*revs_per_mi/60

    Where Gear final is your final gear and GearX is the gear ratio you are calculating for.
    Jared Cromas
    SCSCC Race Steward / Pointskeeper
    '12, '11, '10, '08 ITA Champion
    2008 Driver of the Year
    #111 ITA 1990 Gold Acura Integra

  3. #3
    Join Date
    Jan 2015
    Posts
    0

    Default Re: Manually calculating gear ratio inputs

    Quote Originally Posted by mc-integra111
    I get 76 in for your tire diameter (=pi()*(235*0.5*2/25.4+15)). If you actually measured 73 in, then use that. The best number to use is the revs/mi number that the tire manufacturer provides. If you don't know this number, see if Tire Rack has this number for your tires. If you still can't gets revs/mi, then you can calculate it.

    revs/mi = 5280*12/Tire Circumference

    Once you have the revs/mi number for the tire, you can calculate the required ratio as follows.

    ratio = GearX*GearFinal*revs_per_mi/60

    Where Gear final is your final gear and GearX is the gear ratio you are calculating for.
    Excellent. Although I'm totally math challenged, I think I can figure this out now. TireRack did have the revs/mi for my tire
    so that makes it easy.

    Thanks for the help!!!!

    J

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