how to tell gear ratio

Discussion in 'Differential & Driveline' started by 86chev, Mar 7, 2012.

  1. 86chev

    86chev Full Access Member

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    hello everyone, im new here and i have a newbie question. So i recently picked up a 77 chevy 3/4 ton 4x4 and was wondering how to tell the gear ratio. the guy i bought it from said they were stock but who knows. also, how do you check to see if you have a locker/limited slip/posi? sorry for such beginner questions but id like to learn more before i start handing out cash for other parts.
     
  2. HotRodPC

    HotRodPC Administrator Staff Member

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    Welcome to GMSB Jesse. First suggestion I always make is, look in the glove box for your RPO code sheet. IF it's not there, and even if it is, and you want to verify it, there is a mathematical way to figure it out by jacking up one wheel, spinng the wheel one time and counting the turns on the driveshaft. I can tell you, the 4.10 was the most common gear ratio in the 3/4 ton trucks. But we can help you ID it for certain if the RPO code sheet is missing. Nice, another K20 owner. Any pics of it?
     
  3. smurph20

    smurph20 Full Access Member

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    LOL my rpo code in my glove box says 373 but the tags on the diffs say 410 gues i got an UPGRADE lol
     
  4. johnmc

    johnmc Full Access Member

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    pop the cover drain the fluid count the ring gear divide it into the pinion gear . There it is you ratio. Then you will know if you have a locker or posi unit and you will have done your oil change on the rear end for about 20 .00
     
  5. 86chev

    86chev Full Access Member

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    thanks for the info. no pictures yet and i cannot check the RPO sheet because it is actually an 86 2wd body sitting on a 77 K20 frame. when i return home from college i plan on changing all the fluids so i will have to check then. Is there an optimal gear ratio to run 38s? i have a stock 350 with turbo 350 trans and dont want to lose too much power by running 38s with my current gearing.
     
  6. crazy4offroad

    crazy4offroad Full Access Member

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    4.10 if you want better highway rpms, 4.56 if you plan to offroad it more.
     
  7. HotRodPC

    HotRodPC Administrator Staff Member

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    BTW, there is a way to tell what ratio you have wo removing the cover. It involves jacking up the truck and counting turns of the Driveshaft to 1 full wheel revolution. If interested, let us know. Either I or someone else who knows how can explain in detail.
     
  8. johnmc

    johnmc Full Access Member

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    I would like to know this formula . if anyone knows it.I dont know how accurat it is.
     
  9. johnmc

    johnmc Full Access Member

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    we are running 4.10 gears with 35 boggers with a turbo 350. It is great back road driving but on the highway 65 mph the rpms are around 2,250 for me its buzzing a little bit. But with 38's 4.10 would be better and have lower rpms
    As for off road if you are bogging I would want 4.56 or more with 38's
     
  10. crazy4offroad

    crazy4offroad Full Access Member

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    I thought it worked like this...

    Count the number of times the d-shaft spins in 1 revolution of the tire. Close to 3 turns = 3.08. About 3 and a half turns = 3.42, a little less than 4 turns = 3.73, a little more than 4 turns = 4.11, and 4 and a half turns = 4.56, etc.

    But I dont think it really works that way.
     
  11. HotRodPC

    HotRodPC Administrator Staff Member

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    You are pretty much on target C4 with one exception. You are 100% correct on a locker where you'd also have to jack up both wheels and the clue here is IF BOTH WHEELS TURN THE SAME DIRECTION. So if both wheels turn the same direction, then you are right on the money, except that I like to say if its 3 3/4 turns which in a fraction is 3.75 hence 3.73 gear ratio rather then saying a little less than 4 turns. Again, C4's method is correct IF THE WHEELS ARE LOCKED UP AND TURN THE SAME DIRECTION.

    If your wheels spin in opposite directions, like an open differential or a G80 Eaton Gov Lok, then you will multiply the turns on the driveshaft by 2. Reason being, the spiders are spinning on the other side spider gear dividing your turns by 2. (This of course with 1 wheel jacked up and 1 on the ground)
    So for an easy one to start with, IF your driveshaft turns a hair over 2 times, (to be exact, it would be 2.05 times) So 2 turns, x2 = 4.00 so safe to assume 4.10 ratio (4.11 if that is the ratio your rear end uses). If driveshaft turns a haiir over 1 1/2 turns (1.54 to be exact) x2 = 3 and safe to assume 3.08 gear ratio. driveshaft turns little under 1 3/4 times, x2 = 3.50 save to assume 3.42 ratio. It's very very easy to calculate guys. If you don't get it, just jack it up count your turns, and pay close to attention to little under, or little over, post it up and I can tell you what you have. Never been wrong yet. :High 5:
     
  12. 86chev

    86chev Full Access Member

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    ok so i jack the rear diff up, put it in neutral, and turn. then if i turn one wheel and the other turns the other way it is an open diff? if the both turn the same way it is a limited slip/locker? if they both turn the same direction then i count the number of revolutions of the driveshaft with one complete turn of a wheel with both tires off the ground. if the wheels turn different ways i only jack one wheel up then turn the wheel i time and multiply the number of revolutions of the driveshaft by 2. does this work for both front and rear? if yes, are the rules the same for the front? thanks guys
     
  13. HotRodPC

    HotRodPC Administrator Staff Member

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    You got it. Same rules apply for front diff too.
     
  14. ChevyN00B

    ChevyN00B Junior Member

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    Thank goodness I found this place. I recently bought a 1977 k10 and think my ratios between the front and rear diff are different. When I go into 4 wheel after a little distance my truck feels like it's fighting with itself to just move. I am a total newbie but always had love for the old school bowtie. I know this is an old thread but i was stumped as to how i was gonna figure out my ratios total NOOB. Appreciate all the info here and look forward to getting to know what I'm working with. Aloha
     
  15. ChevyN00B

    ChevyN00B Junior Member

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    Ohh if anyone sees this doing the tire/shaft rotation formula Does the size of the tire matter when doing the test?
     

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