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Any guesses on the ratio?

Discussion in 'Differential & Driveline' started by mr4454, Jul 13, 2019.

  1. mr4454

    mr4454 Member

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    Hello all,
    I have a 12 bolt rear end I took the cover off and counted 40 teeth on the ring but was unable to count the pinon. any ideas what my gear ratio would be? Hope the photos will help. its 1978 k10 4x4
    Thank you all, and have a great weekend

    IMG_2970.JPG

    IMG_2971.JPG

    IMG_2969.JPG
     
  2. Arkansas_V8

    Arkansas_V8 Proud Redneck

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    Just a guess. 3.08

    Arent they usually marked somewhere with the gearing?

    Check the outer part of the ring gear for the numbers. Some are marked.

    Sorry been a while since I have done it.
     
    Last edited: Jul 13, 2019
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  3. Arkansas_V8

    Arkansas_V8 Proud Redneck

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    If its marked it will be to the left of that yellow line on the ring. 20190713_204629.jpg
     
  4. legopnuematic

    legopnuematic Supporting Member Supporting Member

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    3.07 is 14:43 [​IMG]

    Sent from my Moto Z (2) using Tapatalk
     
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  5. mr4454

    mr4454 Member

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    awsome thanks for the info, if it's not marked then what do I do?
     
  6. Arkansas_V8

    Arkansas_V8 Proud Redneck

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    Pull the ring, and count the pinion I suppose.
     
  7. CSFJ

    CSFJ ----------------- Supporting Member

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    The picture you posted shows 14:43. Divide the 43 by 14 and you get 3.07xxxx.
     
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  8. Arkansas_V8

    Arkansas_V8 Proud Redneck

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    Legopnuematic posted that to show mr4454 where the numbers may be. If he saw a picture I didn't with numbers, then I apologize.
     
  9. CSFJ

    CSFJ ----------------- Supporting Member

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    Lol. I wasn't paying attention. Though that was the OP's pic.
     
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  10. skysurfer

    skysurfer Full Access Member

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  11. C10MixMaster

    C10MixMaster Full Access Member

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    its an open rear end so

    1 chalk front wheels
    2 jack up 1 rear wheel
    3 pick a reference point on the wheel you jacked up such as a wheel weight or letter on the tire
    4 spin the wheel until the reference point is down
    5 mark the drive line on the bottom
    6 spin the drive line counting the revolutions of the drive line until the TIRE has gone around TWICE and is in its original starting place
    7 if ithe drive line spun
    3 plus a smidge = 3.08
    3 plus 1/4 = 3.23
    3 plus almost 1/2 = 3.42
    3 plus 3/4 = 3.73
    4 plus a smidge = 4.11
     
  12. Keith Seymore

    Keith Seymore Full Access Member

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    The only axle I see available with 40 teeth on the ring gear is a 3.08 (40/13= 3.08)

    https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1978-Chevrolet-Truck.pdf (page 128)

    You should be able to see at least one tooth on the pinion. The trick is to mark it with crayon/paint and rotate the pinion slowly, counting until that tooth comes back into view. (You won't be able to see all the pinion teeth at the same time).

    K
     
    Last edited: Jul 15, 2019
  13. Richard Schultz

    Richard Schultz Junior Member

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    If you have a 305 motor,3.08s.Look in the glove box,should be a code build sheet in there.
     
  14. Keith Seymore

    Keith Seymore Full Access Member

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    305 cu in engine not available in a '78 K10, per the link I attached.

    While checking the SPID label is an excellent idea, if it is the base ratio then it won't help much. It will show as "GQ1 - Base axle" but not define what ratio that is (because it changes, based on the powertrain charts).

    If it is an optional ratio then it will go ahead and give the RPO and the corresponding ratio (ie, "GT4" - 3.73 axle").

    K
    78 k10 fed.JPG

    78 k10 Cal.JPG
     
    Last edited: Jul 16, 2019
  15. 4WDKC

    4WDKC Full Access Member

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    these also assume the axle hasnt been swapped.. getting the numbers off the ring gear is the best option.
     

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