Strange Rear End Gear Ratio

Discussion in 'Differential & Driveline' started by CalgaryBiker007, Aug 10, 2017.

  1. CalgaryBiker007

    CalgaryBiker007 Junior Member

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    Hello group. I have a 77 GMC 1500 with what appears to be a 10 bolt, 8.5" rear end (the truck style with the 12 bolts on the cover, and the irregular shape - photo attached). I ordered a rebuilt kit for it as the bearings appear to be needing replacement, and thought I would check out the gear ratio as well in case I pull it apart and find I need to order a new ring and pinion for it also. In my test, 1.9 full turn of the drive shaft yields exactly 1 rotation of the rear tire. This seems like a crazy high gear ratio. I am thinking someone put in an unorthodox mix of ring and pinion gear to achieve this? Has anyone else ever found such a high ratio from stock? The truck has factory cruise control on it, so someone at some point in the last 40 years did a lot of highway driving with it. I measured the ratio 3 times thinking I made a mistake, but it was the same every time (just under 2 revolutions of drive shaft to yield 1 tire rotation). Any insight on this would be appreciated.

    20170707_063905.jpg
     
  2. WFO

    WFO Full Access Member

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    I recon it's a 3.73. You need to hold one wheel still while counting revolutions. The other wheel was likely going almost 1.9 revolutions in the opposite direction.
     
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  3. Honky Kong jr

    Honky Kong jr Full Access Member

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    That's a 12 bolt.....not a 10 Bolt. If you ordered a kit for a 10 bolt only 1 bearing will fit ........the outer pinion bearing. If you pull the cover the ratio should be stamped on the ring gear. If you find it and don't understand it post a pic and we can do the math for you and post up your ratio.
     
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  4. Sublime

    Sublime Full Access Member

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    And i believe you double the rotation count on an "open" (both wheels spin oposit or only one wheel spins)
     
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  5. Jrgunn5150

    Jrgunn5150 Full Access Member

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    Agreed.

    Also, you can't mix and match ring's and pinions, the pitch changes with each set.
     
  6. Zoomalot

    Zoomalot Junior Member

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    I am thinking this is not a 8.5" 10 bolt diff.
    Needs 90 degree lugs at 5 and 7 o'clock carrier position to be a 8.5". And I count 12 bolts.
    Count teeth in ring and pinion and follow charts for ratio.
    Few parts will interchange
     
  7. Mobius

    Mobius Junior Member

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    I don't know much about the interchangeability or identification of 10/12 bolt parts, but I do know that one of the other posts is correct about the ratio: if you have an open carrier, you need to double the number to get the correct ratio. Sounds like you have a 3.73.
     
  8. MikeB

    MikeB Full Access Member

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    Passenger car 12-bolts used 3.31. Truck 12-bolts used 3.08 and 3.42, and I'm not sure there was anything in between -- maybe a 3.23?

    12-bolt rear ring gear size is 8.875", not 8.5".

    You will find some stamped numbers and letters on the front of the passenger side axle tube. More than likely you will need to remove surface dirt and rust to read them. This may not be for a 12-bolt, but it's an example. Of course, someone could have changed gears in the past.

    [​IMG]
     
    Last edited: Aug 16, 2017 at 11:59 AM

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