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rear 12 bolt gears

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by blaze4999, Aug 27, 2006.

  1. blaze4999

    blaze4999 Registered Member

    Feb 1, 2006
    Likes Received:
    Quantico, VA
    I had my truck on the lift today working on it. I was trying to figure out what my gear ratio is in the rear diff. I turned the tire over so that it would turn the driveshaft. I counted the rotations and it doesn't seem right. I counted something like a rear ratio of like 1.8:1 Is this possible? Someone please explain what the hell is goin on. I thought it was a stock 12 bolt from a 1974 k5 blazer. Am I wrong?
  2. southernspeed

    southernspeed 1/2 ton status Premium Member GMOTM Winner

    Dec 24, 2003
    Likes Received:
    You've got to either jack it up and turn both wheels in the same direction, jack one wheel up, turn it and count the prop turns then halve it or drive forward and have someone count the prop turns. Jacked up turning one wheel will turn the oposite one the other way unless you have a locker giving an inaccurate reading.
    I'd say you've probably got 3.52 or there abouts. Try it again like I suggested.
  3. bigblock72

    bigblock72 1/2 ton status Premium Member GMOTM Winner

    Oct 18, 2005
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    American Fork, Utah
    If you spun just one tire then the gears in the differential were turning as well, so you have to factor that in as well. It's easy though since the ratio is 2:1. Just multiply your finding by 2 and you something close to your answer which is most likely going to be 3.73. It's hard to get both rear tires on an open diff to spin at exactly the same speed, the way you did it is the easiest and most acurate way to do it without pulling the cover off.

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