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Gear Ratio Equation?

Discussion in 'The Garage' started by SchnorrCS, Dec 4, 2005.

  1. SchnorrCS

    SchnorrCS 1/2 ton status

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    Hey Everybody-
    I have heard that there is an equation for figuring out what gear ratio is in your axles. It's something like rpm times speed divided by 4 or something. Anybody know it?
    Thanks yall
    cS
     
  2. sweetk30

    sweetk30 professional hooker Premium Member

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  3. SchnorrCS

    SchnorrCS 1/2 ton status

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    That's cool, appriciate the help and all, but I thought there was an equation for figuring it out with out pulling the cover.
    cS
     
  4. AussieK5

    AussieK5 1/2 ton status

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  5. SchnorrCS

    SchnorrCS 1/2 ton status

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    anyone ever hear of a 5.25, 5.16, or 5.08 gear ratio in the rear end of a 74 K5 with what on first inspection I thought wasz a Corp 12, but now I may have to go look again. I know it has 12 bolts on the cover and 6 lug axles, but it also seems realllly strong, cause I've beat the s out of it and she keeps on ticking.

    Thanks for the help
    cS

    Also, given the description I gave, anyone have any idea of what kinda weirdo axle I have?:eek1:
     
  6. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    here's my thoughts: either you calculated your gear ratio wrong, or someone has installed after market gears. in regard to what kind of axle you have - sounds to me like you got a standard 12bolt.

    colby
     
  7. SchnorrCS

    SchnorrCS 1/2 ton status

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    Well, that's what I thought as well, but after double checking, I keep getting the same thing. I guess I'm gonna have to pull the cover. Damn, I wish I would have thought to count 3 weeks ago when I was under there changing gear oil anyway.
    On a side note, is it just me or is gear oil the nastiest stuff on the face of the planet, or what?
     
  8. Dallin

    Dallin 1/2 ton status

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    5.13 is a common gear ratio. If you did everything right than that is what you have.

    Is your speedometer correct? Check by doing 60 MPH and time yourself between mile markers. Use longer distances like 10 miles to increase the accuracy. If your speedometer isn't exact it will mess up your gear calculation.

    If this method doesn't work than jack up a tire, spin it twice (if your unlocked) and count the number of driveshaft revolutions. That is your gear ratio. Common raitos are 3.40, 3.73, 4.10, 4.56, 4.88, 5.13

    Pulling the cover is really a waste of a gasket if all you are trying to do is figure out the gear ratio.
     
  9. SchnorrCS

    SchnorrCS 1/2 ton status

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    I know my speed is right. I used a GPS. I may be off by 10-20 on the RPMs, but not off by that much.
     
  10. Bullitt427

    Bullitt427 Registered Member

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    It's simply:

    RPM/336 X Tire Diameter / MPH = Ratio

    Does you mom still teach 4th grade Clint?
     
  11. zero cool

    zero cool 1/2 ton status

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    I'd write it out as:

    (RPM * Tire Diameter) / (MPH * 336 * overdrive ratio) = Axle Ratio


    Or, for those with less math savvy:

    Get yer calculator and enter the RPM

    Hit the "x" key

    enter the Tire Diameter

    Hit the "divide" key

    enter the MPH

    hit the "divide" key

    Enter 336 (or 355 if you're using a non-lockup transmission)

    If you don't have overdrive, hit the "=" key.

    Otherwise, hit the "divide" key

    Enter your overdrive ratio (0.7 for the 700R4/4L60, 0.75 for the 4L80)

    Hit the "=" key.


    Or just go to http://www.grimmjeeper.com/gears.html, plug in your transmission and tire size and try out the gear ratios until you find the one that fits best. :D
     
  12. SchnorrCS

    SchnorrCS 1/2 ton status

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    Who is this?
     

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