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Final RPM ratio calculation

Discussion in 'The Garage' started by Bullet, Dec 13, 2003.

  1. Bullet

    Bullet 1/2 ton status

    Mar 14, 2001
    Likes Received:
    Abilene, Texas
    For the sake of my laziness...
    Currently I run a 700R4 (3.06) NP241 and One tons with 4.56 gears to spin my 37's right now to go down the highway at 70 the 383 turns over 2000rpm

    I plan to swap to 5.13 gears and 38's can anyone tell me if how far or close I will be to my old ratio? I lokked at the chart at 4Lo.com and its a little help, but is there any better exact way to finding out what my REVs are going to be? Thanks /forums/images/graemlins/thumb.gif
  2. sled_dog

    sled_dog 1 ton status

    Sep 3, 2002
    Likes Received:
    Austin, TX
    final drive ratio on the setup you have now is 3.19, with the new setup would be 3.59. At 3,000 RPMs in overdrive now you should be able to do 103, with the new setup would be 94. Personally I'd try the 4.56s and 38s and see how it is. I don't think I'd go with 5.13s unless you want some crawl ratio, even then the crawl ratio doesn't jump much between the two.
  3. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

    Jul 23, 2000
    Likes Received:
    E-town baby!
    Here Matt

    Speed (mph) x gear ratio x tranny ratio x 336 (355 for non lock up auto's) all divided by the tire diameter

    75 x 4.56 x .70 x 336 / 38= 2116 rpm

    75 x 5.13 x .70 x 336 / 38= 2460 rpm

    The bigger tires are heavier so a little extra gear helps get the truck moving like it did with the smaller tires. The lower gears also make things a little easier on the 700R-4.


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