Speedometer gear change

Discussion in 'The Garage' started by GruntHunter, Nov 1, 2006.

  1. GruntHunter

    GruntHunter 1/2 ton status

    Apr 28, 2002
    Likes Received:
    Florida (Space Coast)
    I never could understand why my '87 K5 was such a 'dog' until I was getting the rear end checked out and discovered it had 3.08 gears in it. The overdrive was always kicking in and out whenever I'd even drive over a bridge, I suspect because of the high gearing. I can't believe it actually came from the factory like this, but ......... Also, I had about a 9 mph slow speedo error at 50 mph so I knew something had to be wrong.

    Solution: I bit the bullet and just installed Auburn Pro-Series Posi units front and back with Yukon 4.11 gears. I sort of thought the gearing change might get me pretty close back to normal but now I find I have an error of about 12 mph fast at 60 mph. I'm running Pro-Comp 31 x 12.50's on Rock crawler 15x8" rims and strock supension. I'd like to go to 32's or 33's but don't want to lift it.

    Question is: what do I need to get my speedometer reading correctly again. This is my hunting vehicle so it's not a big deal except that I have a special insurance rate for it if I don't put on more than 7500 miles a year. Now I actually have a 21 mph delta from what I had before so I need to get it changed.

    Anyone out there running the same gears and tires that can give me a part number or ? ? ? for the speedo gear ?
  2. goldwing2000

    goldwing2000 Guest

    You can fit 33s without a lift. At most, you'd have to trim about an inch off the wheel openings.
    That would get you back in the ball park of the right speed.
  3. randy88k5

    randy88k5 1/2 ton status

    Nov 8, 2004
    Likes Received:
    Lynchburg, VA, USA
    I posted this in a previous thread ( http://coloradok5.com/forums/showthread.php?t=176781 )


    OK, I will describe how to do it. This is not the only way, but it is the way I would probably do it.

    Note: this is not really a "calibration" like you would find on much newer computer controlled cars. Those you can just plug into a computer and reset your tire/gear size and it will read correctly. You have to physically change the small plastic gears to get the correct reading with our rigs.

    First, you need to know your exact tire diameter. Get the measuring tape.

    Find your current gear ratio. Actually count the teeth or use the driveshavt vs. wheel spin method.

    Know what transfer case you have.

    Now, you have to count the teeth of your current drive and driven speedometer gears. Find where the speedometer cable goes into. It looks like this:

    Now remove the hold down bolt and gently rock the sleeve loose from its hole. When it comes out, it will contain the driven gear, which looks like: [​IMG]

    count the teeth.

    If you look into the hole, you can see the drive gear:
    [​IMG] [​IMG]

    count the teeth.




    Now, using the calculator (speedometer rev/mile being 1000) and resource page I posted above, find out which combination will work for you to give you the correct reading on the speedometer face. You might need to swap both drive and driven gears.

    Images and some good info found at http://www.transmissioncenter.net/sp...n_______va.htm in addition to what I posted above.

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