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How do you recal the speedo on an '86?

Discussion in 'The Garage' started by AJMBLAZER, Dec 17, 2006.

  1. AJMBLAZER

    AJMBLAZER Better to be lucky than good. Premium Member

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    Big Ugly is, as the sig shows, a '86 Chevy K30 M1008 CUCV. 6.2L diesel, TH400, NP208, 4.56 gears, and originally it had some itsy bitsy tires that MIGHT have been 31" tall. The military wanted them to tow and haul, not go fast. Now it's got some 36" HMMWV tires on it and the speedo is over 10% off. I've checked with my GPS and it's somewhere around 13%.

    How's a guy supposed to fix this? I know I have a speedo cable and a speedo gear in the transfer case...different speedo gear?

    Never had to do this on an older GM before.
     
  2. sandawgk5

    sandawgk5 3/4 ton status

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    Yep the only way to do it is to change the driven gear in the Tcase tailshaft housing. If you pull the cable, there will be a small bolt that holds a housing in the tailshaft. Remove the bolt and pull the housing it may be a tight fit as it has an oring on it. Once you remove the housing you can see the driven gear attached to the housing and the drive gear on the tailshaft inside the case. The gears are color coded based on tooth count.

    Here is a site that has a lot of info on speedo gears.

    http://www.transmissioncenter.net/speedometer_calibration_______va.htm

    And here is a good calculator to determine what gear you need.

    http://www.tciauto.com/Products/TechInfo/speedo_gears.asp

    Ira
     
  3. trailblazr81

    trailblazr81 1/2 ton status

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    +1 Yup, had to do this on my 81 and 86 K5.
     
  4. AJMBLAZER

    AJMBLAZER Better to be lucky than good. Premium Member

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    Yep, sounds exactly like what Ford and Suzuki did.

    So uhm, where's a guy buy new speedo gears? Does GM still carry them?
     
  5. AJMBLAZER

    AJMBLAZER Better to be lucky than good. Premium Member

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    Okay, so I was wrong, it's a bit more complicated...

    I need a drive gear, driven gear, AND a housing? or can I reuse the housing I have?

    It'd be in the transfer case correct? The first link mentions that the NP208 uses the same peices as the TH400.
    http://www.transmissioncenter.net/speedometer_calibration_______va.htm#400
    I obviously need to pull out what I have now and figure out how many teeth it has, huh? I need that for the equation that TCI has. That's what I had to do on my Ranger.
     
  6. trailblazr81

    trailblazr81 1/2 ton status

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    I got mine at Napa. Figured out how many teeth the gear needed and they looked it up in a catalog.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Depending on the drive/driven, a new "sleeve" (what you called the housing?) can be required, as the gears are offset due to size differences.
     
  8. AJMBLAZER

    AJMBLAZER Better to be lucky than good. Premium Member

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    Okay, gotcha.

    Do the drive/driven gears match with each other (35 and 35) or do you mix and match those (35 and 42)?
     
  9. sandawgk5

    sandawgk5 3/4 ton status

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    It is a mix and match. The first link I posted list the 2 different sleeves or housings required for the driven gear. If you use the wrong sleeve for the driven gear it will only last aboot 5 miles then all of the teeth get ground off of the gear:doah:. Don t ask me how I found this out. The drive gear will be something like a 15 tooth and the driven can be anything from like a 35 up to aboot 42 or something like that.

    Ira
     
  10. AJMBLAZER

    AJMBLAZER Better to be lucky than good. Premium Member

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    So uhm...forgive the questions but here's another. This is more complicated than the Fords and Suzukis, bear with me.

    With the driven gear, do you put it into the equation and figure out which tooth count to change it to or are you only supposed to change the drive gear? Or vice versa?
     
  11. sandawgk5

    sandawgk5 3/4 ton status

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    You can change them both but the driven gear is easiest. To change the drive gear you have to disassemble the tail housing and slide the gear off of the shaft. To change the driven gear you pull the sleeve with one bolt and slide the gear out put a new one in and put the sleeve back in the tail housing. Then you line up the number that corresponds to the tooth count of the gear with a mark on the case. I do not know how to do Ford or zukes but I dont see it getting any easier than that. If you go to the second link (the TCI link) there is a calculator that you just plug in the numbers and it will tell you the driven gear based on your tires gear ratio and drive gear tooth count.

    Ira
     

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