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speedo - search not working

Discussion in 'The Garage' started by mccomas, Jan 21, 2004.

  1. mccomas

    mccomas 1/2 ton status

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    search is not working - i saw some posts a while back ago about speedo gears. i am running some 255 tires for the next several months and my speedo is way off. my dodge ram had a little gear that i was able to change and you could get it with different numbers based on tire size. i am taking a long road trip this weekend and need to get the speedo straight. can someone give me a location / instructions if there is such a gear that i can change - i also need a recommendation on a speedo gear size for my 255s. thanks...
     
  2. mechted

    mechted 1 ton status Premium Member

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    what kind of transmission? gears?
     
  3. mccomas

    mccomas 1/2 ton status

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    i have a 700-r4 and 4:10 gears
     
  4. mechted

    mechted 1 ton status Premium Member

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  5. mccomas

    mccomas 1/2 ton status

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    i went to the calculator and determined that i need a 15 tooth drive gear and a 41 tooth driven gear. i assume that i have a non electronic speedo cable in my 86 700r4 - is this correct? if so, i have a part number for the 41 tooth driven gear. it looks like the drive gear is a bit more complicated to change and i am not finding any info for changing it. i hope that it came stock as a 15 tooth - if so, i will be good to go.
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    Cable drive speedo, yes.

    Combinations of drive/driven (speedo) gears were HUGE, and to be honest, without pulling the drive gear, I don't know as there is any way to tell for sure. Don't know how GM figured out which drive/driven gear combo's to use, but there is literally an entire page of possible drive/driven gears for the '85/86 C/K series.
     
  7. mccomas

    mccomas 1/2 ton status

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    based on my rig coming stock with 31 inch tires and 3.08 gears, i assume that my drive (the hard one to change) gear is something around 19 tooth. from what i found on- line last night, the drive gears are color coded as well. 15t = gray; 17t = red; 18t =blue; and 19t = yellow. i am going to pop it apart tonight and hope for gray but i do not think it will be there. after doing several google searches using "speedo gear change 700r4" i found a lot of info on the speedo gear change... i am going to play around with the driven (easy one to change) gear. right now i just need to get something that is manageable. at 60 mph my gauge is pegged all the way to the right - that can not be good for the gauge... when i get back next week, i am going to research changing the drive gear. my 255s and 4.10 gears will use the same drive gear as 36s and 4.88s. so with this change my drive gear will be good to hook. it looks like i will have to pull the transfer case and the tail shaft of the xmission - probably not hard but i have never pulled a transfer case down and i am a little apprehensive....
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Why will you need to pull the transfercase off? The speedo stuff in 4wd is on the t-case output.

    Some people don't like going by color as they say that can change with age, but I've never actually seen that happen. FWIW, if you get the gears off, 99% of the GM speedometer gears I've seen have the tooth count stamped right into them, on the "edge" that rides up against the output shaft when the gear is installed. Small numbers, but it's typically there.
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    Also, there are (probably) hundreds of "ratio adapters" that GM used over the years, junkyard scrounging will usually net you a few.

    For isntance, one I've got is something like 1.3:1, and it just threads onto the transmission/t-case in place of the cable, and the cable threads onto it.
     

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