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Info needed on speedo corection

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by panicx, Oct 18, 2002.

  1. panicx

    panicx Newbie

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    83 K5 4x4 speedo reads way to fast..
    It has been changed to 350/th350 308?
    31in tires rear diff ratio ?
    Oh, how do i do that to ? jackup - spintire -count ? what ?
    Any info or link would help, Thank you
    Kelly
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Pull off the diff cover and look for the tooth count that is stamped in the edge of the ring gear. (big gear) Do a search on the board, its been covered what numbers you use quite a few times. Forces you to change the fluid and inspect the innards anyways, which is a good idea.

    As to gearing, I don't have any links to speedo gear info really, and the fact that the speedo gear is in the t-case doesn't help either. I have almost NO experience with that setup, so I wouldn't even know what the 208 speedo gear stuff is from...TH400? TH350? (and IIRC, there are 3 plastic speedo gears in a 208??) If someone can answer those, then you can probably figure out (or get close) with some of the online calculators, and also by figuring how much % off you are.

    Friend says that the calculators used online are typically not accurate, and he's shown me an example that proved it (compared GM's actual speedo gear usage to what the calculator spit out) so I'd go with the % off method. But you gotta know your axle gearing, and how fast you are going compared to how fast the speedo SHOWS you are going. (either get paced by someone that has a correct speedo, or time yourself at a speedometer check area, etc)
     
  3. txbluethunder

    txbluethunder 1/2 ton status

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    there is two speedo gears in the 208 (driver and driven). i've never worked with the th350 though...only the 700r4 so i really couldn't help you. if you do find an answer make sure to post it so we can find it later...
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    I almost posted about the 700/350 differences, but I don't think that matters. GM speedo cable always turns at 1000 (or is it 1001? : ) revolutions per mile, so the only factor is what gears the t-case takes. Which I'm not sure of.


    Only reason I had a question on how many speedo gears there were in the 208 was because I found an exploded 208 in the 'yard, and it had 3 different plastic gears. Driven of course, but appeared to be two different drive gears. (all were plastic of course) I've got them laying around somewhere...
     
  5. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    A lot of tranny shops sell 'Ratio boxes' that correct speedo error. You still have to know how far off the speedo is though.

    Rene
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    GM made and used these. Fairly common on the earlier vehicles, but they are still listed in the '85/86 GM parts manual. I've got a few around, even have one on my truck. Totally guessed and threw it on there when I was building the truck, as I had it lying around. Got lucky, speedo is now dead on.
     
  7. rcpilot

    rcpilot 1/2 ton status

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    dyeager535 is correct 1001 rev per mile for GM... visit United Speedometer ... they can send you a new driven gear or better yet a ratio box.. ratio boxes are around $65.00... just do the follow the test insrtuctions on their site... /forums/images/graemlins/thumb.gif /forums/images/graemlins/k5.gif
     

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