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Speedometer Question

Discussion in 'The Garage' started by lectric80, Apr 9, 2007.

  1. lectric80

    lectric80 3/4 ton status

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    I am sure somewhere in here someone has gone over this, but I am not seeing it.

    I have an 80 Blazer running 33's with stock suspension, 3.08 gears, 350, and a TH-350 mated to a NP205. I use this mainly as a driver with the weekend off road adventures pretty limited. This truck will be going off road more as I get more done that I want to do. What I need is to find anyone who has been able to correct the speedo using tires this much larger than the stock 235's. Any help would be appreciated.
     
  2. boz42

    boz42 1/2 ton status GMOTM Winner

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    it can be done, call a place like randy's ring & pinion & buy a new speedo gear. they can help u select the right one, to get your speedo back close to stock.

    also there are gear boxes that go inline between the speedo gear & speedo cable that came stock on some trucks, they mount just like the speedo cable does. they can change your speedo up or down by 5-10 miles an hour. the trick is finding one that changes it the right amount.
     
  3. vortec

    vortec 1/2 ton status

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    call me lazy, but when i go from my 31s to 33s, i plan to just leave the speedo gearing as-is, and know it's not dead-on. but, this isn't my dd, so you may want to actually do it right.
     
  4. RootBreaker

    RootBreaker 3/4 ton status Premium Member

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    at least you automatic guys can get yours fixed with the drive or driven gear...

    us stick guys dont have that option... I just use my gps.... :crazy:
     
  5. boz42

    boz42 1/2 ton status GMOTM Winner

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    why not the speedo gear is in the t-case, u can change that on yours too?
     
  6. Currank5

    Currank5 1/2 ton status

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    Go to any GM dealership and you can get a new gear to put in the t-case. When i went they had a chart with what gears/ratios etc. They were all color coded. They're cheap so you can get them and exchange them until you find out if you need to adjust higher/lower or whatever until you're dialed in to where you need to be.
    At the dealership i went to they were pretty helpful.
     
  7. Krazy Kid

    Krazy Kid 1/2 ton status

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  8. RootBreaker

    RootBreaker 3/4 ton status Premium Member

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    wow I need to call them.. didnt think I could correct mine....
    I have the ratio adapter on my truck now..

    I have a smc465 matted to a np205. I have the ratio adapter that screws onto the xfer case.... then my cable to it (cable went bad so need a new one)

    didnt know I could change out the drive gear in a np205.... anyone do this?
    I will call them to see....
    pic sucks as it is my camera phone.... but i do have the ratio adapter.... just may need a different one...
    [​IMG]

    this adapter is stock and was to assist in fixing the speedo.. it has written on it
    SW in big print
    W9 on one side
    777F .9743D on the other side....
     
  9. lectric80

    lectric80 3/4 ton status

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    I see, so when I put in the new speedo cables today, if I look at the color of the current gear, I should be able to call the local dealer and get a different color gear to adjust the speedo closer to stock. It doesn't have to be perfect, just a little closer than showing 45 when I am doing closer to 65.

    Root Breaker, I do know that the speedo in all four wheel drives runs off the t-case so that even when you are in low range it still reads correctly.

    Thanks for all the info guys.
     

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