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Thread: How fast is the RPM at speed, or the speed at RPM?

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    Super Moderator bmorgil's Avatar
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    Dec 2018
    Northwestern Ohio

    How fast is the RPM at speed, or the speed at RPM?

    Here is a way to figure out just how "fast" everything is going. This is a great way to determine your "crawl speeds" and top speeds, vs RPM or vise versa. I have the T90 and Dana 18 transfer case ratios below.

    As an example, my CJ3A crawls at 1.6 mph at 700 rpm in low range first gear. Multiply the transfer case gear ratio you are in times the transmission gear ratio you are in. Use this number as the transmission gear ratio number in the calculator.

    T90 Transmission Ratios:
    First 2.798 to 1
    Second 1.551 to 1
    Third 1.000 to 1
    Reverse 3.798 to 1

    Dana 18 Transfer Case:
    High 1.00 to 1
    Low 2.46 to 1

    CJ3A Rear Axle ratio:
    Last edited by bmorgil; 11-30-2019 at 07:56 AM. Reason: Clarify

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