Gear ratios

Discussion in 'Tips 'n' Tricks Topic' started by Dennis Wilkie, Sep 15, 2020.

  1. Dennis Wilkie

    Dennis Wilkie Veteran Member

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    I know there's really no way to tell without opening it back up, but can anybody give me an educated guess on what rear end gear ratio I have? It's a 10-bolt GM rear end with a NP 241-C transfer case and 700-r4 in a 1992 k1500 extended cab. It's supposed to have 3.42's in it, but the rear end was clearly swapped out along with everything else from bumper to bumper. I tried looking up the numbers I got off of the axle when I changed the pumpkin gasket, but didn't come up with anything. All I know for sure is with the speedo pegged out at 90mph, it's only running around 2500rpm.
     

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  2. Coadster32

    Coadster32 Veteran Member Lifetime Gold Member

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    Jack up the back...put it in neutral...put a small piece of tape on the driveshaft and the tire, or mark them somehow. Spin the tire and one full revolution, and count how many times the driveshaft goes around. Pay attention to the last rotation, as it won';t be a full one. This will tell you your ratio. 1/2 turn =3;42...3/4 turn=3.73, etc.
     
  3. jeff swisher

    jeff swisher Veteran Member

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    Jack up 1 rear tire and spin it 2 revolutions with the transmission out of park.
    Count drive shaft revolutions and that is your ratio.
    If it is a posi track or a spooled rear end then both tires need to be off the ground.
    If you spin 1 tire clockwise the other will go the same way in a spool or a posi unit.

    Spin that setup 1 tire revolution and count drive shaft revolutions and you have the ratio.
     

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