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Speedometer re-calibration on old school chebbies

Discussion in 'The Garage' started by RootBreaker, Nov 16, 2004.

  1. RootBreaker

    RootBreaker 3/4 ton status Premium Member

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    aight.. today I am going to get my speedo cable... I know that it will be off.. what gear do I need and where can I obtain it to fix my gauge?
    sank y-u
     
  2. rcpilot

    rcpilot 1/2 ton status

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    Try Speedometer shop /forums/images/graemlins/thumb.gif You can call them and they are very helpful or use the links on their site to determine what gear you need..
     
  3. randy88k5

    randy88k5 1/2 ton status

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    Well, its not extremely easy as the new computer ones are, if u got the equipment. Your speedo cable should hook up to your transfer case snout. The snout will have to be removed to ge to your gears. You most likely have 2 plastic gears, one cylindrical one which slips onto the output shaft, and one small circular one that attaches where the cable would go into. You need to figure out the combination of drive (cylindrical) and driven (circular) gears (# of teeth basically). I went through all this trouble, only to find out the plastic gears were not factory made for the combination of tire and gear ratio I was running. The factory gears are color coded for easy identification. I plan on changing stuff around, so I live with it about 8-10 MPH off for right now. This website might help, as it did for me:
    http://www.bgsoflex.com/speedo1.html
    Hope this leads you in the right direction.
     
  4. RootBreaker

    RootBreaker 3/4 ton status Premium Member

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    **** edited
    also a guy at work had a good idea...

    tools needed
    your truck
    extra vehicle
    2 cell phones, walkies or cb's
    stretch of road
    fingers

    ok so you drive your truck... have someone else drive the other vehicle....
    once they are at say 50mph they call you and tell you ... you look at what your speedo says... if it says 60 you go home and open up your dash..
    use your fingers and turn the speedo to say 30mph and bend the needle back to 20.. viola.. 10mph fixed!!!! /forums/images/graemlins/rotfl.gif /forums/images/graemlins/whistling.gif
     
  5. rcpilot

    rcpilot 1/2 ton status

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    Get a GPS for the wheelin...and now you have a new Speedo /forums/images/graemlins/peace.gif
     
  6. BrianDamage

    BrianDamage 1/2 ton status

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    I bought my gear at the dealer for around $7
     
  7. RootBreaker

    RootBreaker 3/4 ton status Premium Member

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    [ QUOTE ]
    I bought my gear at the dealer for around $7

    [/ QUOTE ]

    what gear and how did you figure it out???
     
  8. BrianDamage

    BrianDamage 1/2 ton status

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    [ QUOTE ]
    [ QUOTE ]
    I bought my gear at the dealer for around $7

    [/ QUOTE ]

    what gear and how did you figure it out???

    [/ QUOTE ]

    I went here http://www.tciauto.com/tech_info/speedo_gears.htm


    I looked inside the tailshaft on my transfer case to determine the color of the drive gear (I have 1987 K5, 700R4, np208, and my drive gear was blue). I think you can see it if you pull the spedo gear out. Then I I used TCI's website to figure out what the driven gear needed to be. Checked accuracy witha gps.
     
  9. bshewmaker

    bshewmaker Registered Member

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    [ QUOTE ]
    **** edited
    also a guy at work had a good idea...

    tools needed
    your truck
    extra vehicle
    2 cell phones, walkies or cb's
    stretch of road
    fingers

    ok so you drive your truck... have someone else drive the other vehicle....
    once they are at say 50mph they call you and tell you ... you look at what your speedo says... if it says 60 you go home and open up your dash..
    use your fingers and turn the speedo to say 30mph and bend the needle back to 20.. viola.. 10mph fixed!!!! /forums/images/graemlins/rotfl.gif /forums/images/graemlins/whistling.gif

    [/ QUOTE ] its not a straight 10mph across the board. Your speedometer is off by the percentage of the difference in the height of your tires now and your stock tires. If you had 30's and went to 33's, you would have a 10% difference. So, when you speedometer shows 10 mph you would only be off 1mph, but when it showed 60 mph, you would be off 6 mph. Obvisously, the bigger the difference between stock and the tires currently installed, the bigger the percentage difference. Which gives you more of a discrepancy at any rate of speed.
     
  10. RootBreaker

    RootBreaker 3/4 ton status Premium Member

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    Ok the few shops I have called said they deal with automatics... WHO HERE HAS DONE THIS WITH A SMC465??? I know alot of you own this tranny.. what have you done??/
    a phone number to a shop that can help???
    greatly appreciated


    also If my speedo says im doing 35mph I am actually going faster right? cuz at 35mph on a back road it was pretty scary!!!! /forums/images/graemlins/tongue.gif
     
  11. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    [ QUOTE ]
    I looked inside the tailshaft on my transfer case to determine the color of the drive gear (I have 1987 K5, 700R4, np208, and my drive gear was blue).

    [/ QUOTE ]

    I am pretty sure my other truck , the SM465/np208 and 3.08 originally truck has an orange gear . My 83 with 700r4/np208 and originally 3.08 truck has a blue one .

    Funny thing is I was closer to being right with the 33's and 3.08's than I am with 3.73's and 33's . Cell phone to my wifes car which is 1 year old put me so close at 65 I was happy . The 3.73's are reading around way more than I am going now /forums/images/graemlins/k5.gif /forums/images/graemlins/k5.gif
     
  12. azblazer69

    azblazer69 Registered Member

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    Has anyone ever seen those speedo gear reducers/multipliers? I've seen them a few times years ago... They were like tiny transfer cases that you screwed into the transmission and then you screwed the speedo cable into it... Of course you would need to find the one you needed... I think GM made them... You would have to find out how much your speedo is off by... Using the cell phone/ friend with correct speedo method, you could easily figure out what percentage you are off by...
    If you could find them... I haven't seen one in a while...
     
  13. RootBreaker

    RootBreaker 3/4 ton status Premium Member

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    [ QUOTE ]
    Has anyone ever seen those speedo gear reducers/multipliers? I've seen them a few times years ago... They were like tiny transfer cases that you screwed into the transmission and then you screwed the speedo cable into it... Of course you would need to find the one you needed... I think GM made them... You would have to find out how much your speedo is off by... Using the cell phone/ friend with correct speedo method, you could easily figure out what percentage you are off by...
    If you could find them... I haven't seen one in a while...

    [/ QUOTE ]

    I have an adapter like that.... already... My cable screws onto a small unit that is screwed into my tranny... I cant take pics until monday..... I might just say screw it and get a GPS... /forums/images/graemlins/doah.gif
     
  14. diesel4me

    diesel4me 1 ton status Premium Member

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    Stewart-Warner used to make the gears for those speed reducer boxes that screw onto the transfer case and speedo cable--we used to sell them at the parts store I worked at years ago,they had different ratio's available to correct the spedometer readings.Not sure if they still make them,or if Stewart-Warner is even in business still.A tranny shop might be able to help,they might have some of those boxes and the gears for them hanging around.I've seen those boxes on some 70's GM full size cars as well as 4x4's in the junkyards. /forums/images/graemlins/crazy.gif
     

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