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Tried to get a speedo gear today- Where is it?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by MudNurI, Jun 3, 2002.

  1. MudNurI

    MudNurI 1/2 ton status

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    I went by a Chevy Dealership today, and decided to stop in and see about getting a new gear for my speedo- so I'm at least a little bit closer than I am now..

    First thing he asked me was what color the gear is now that's in it.. WHERE THE HECK IS IT? He said something about the speedo cable, on the end of that, or he thinks that's where it is...he wasn't very helpful. so I explained to him that I now have 3.73 gears, and 33" tires, he says his books ONLY go up to 235's..WELL you know and I know that 33's are a LOT bigger than that. He said he won't be able to get me right on, but should be within 5 or so mph...so what I need to know is where the heck is this gear, and are they REALLY colored? he said it could be blue/red/green/brown, or natural...hmmm I'll go see what I can find.

    UUGGHH
    thanks guys!
    Brandy
     
  2. CaptCrunch

    CaptCrunch 1/2 ton status

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    Brandy

    The speedo gear is in the tailshaft of the tcase. You will see a cable going into it and it will screw off. You will need to pull out the speedo gear and note the color. You also will want to look in the hole and note the color of the sleeve inside. Those, combined with your tire size and gear ratio will be enough info to figure out which set of gears you need to make it accurate. You may need to change the inner sleeve to get it right, but I only needed to change my gear. Also if you have problems working with your local dealer (I know I had some problems with mine getting it right) try these guys
    http://www.speedometershop.com/
     
  3. MudNurI

    MudNurI 1/2 ton status

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    THANK YOU!

    I'll check it out, and see what I find....Probably a BIG suprise..like it will come off easily, wouldn't that be nice..

    I'll check that speedo shop too! The parts guy at this dealership was like 1000000 years old, and was FLABERGASTED when I said I had 33's on it, well, ma'am how did you fit those? I was like well, see its got a 4" suspension and a 3" body lift, so they fit with no problem...the guys chin dropped..specially when I started telling him what it had for gears when it came from GM and what it has now, and blah blah, he thought he was gonna be able to blow smoke up my a** YA RIGHT! heeheheh

    thanks again!
    Brandy
     
  4. CaptCrunch

    CaptCrunch 1/2 ton status

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    typical average parts guy... he knows what the computer tell him. Gotta hope to get one of the gearhead good parts guys/gals for this question. Good luck on the gear... they sometimes can be in there tight as dog snot. Mine was and I just unbolted the tailshaft from the tcase after pulling the dirveshaft. Was real simple and made getting the gear out much easir because I could pry lightly on the back side. It also lets you cahnge that sleeve if needed. Good luck
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    I'm not quite sure why people rely on the gears tat can and do change colors. Its not too common for them to change so much you get the wrong one, but like the ring gears (though not as standard) most GM speedo gears have the tooth count cast into them. It's small, but if you know where to look, more often than not, its there.
     
  6. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    Most of the drivegears in the transfer case are plastic and do not have stampings on them, that is why they are color coded (I have several of the plastic drives and none of them have stampings on them). The piece that rides on that gear and controls the speedo cable is metal (is that the part with the number cast in it?) If so all you have to remove is the cable and the drive to determine what you have (and what you need). Then when you get the parts you may have to remove the tailshaft. Not a bad job, but can be frustrating. Also some trucks came with reductions on them that go between the cable and the drive housing. In a given year most trucks would come with say a 4.10 ratio, but others would have a 3.73. Instead of having to figure out which case goes into which truck (or as an afterthought....GM woops!) they would run all t-cases with the same speedo drive and increase/decrease the speedo cable with a reducer. You might be able to find one that works for you, and I know there is a company on the net that builds them using your original setup and modified setup as parameters for building. I'll see if I can find them online.
     
  7. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    http://fteufert1.home.att.net/trans/speedo.htm

    this is a listing of colors and tooth counts and note:
    Some cars (or trucks) (1970 Oldsmobiles for example) used gear reduction adapters in some situations ranging from .619:1 through 1.202:1
     
  8. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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  9. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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  10. 70~K5

    70~K5 1/2 ton status

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    Some of the 1st gens came with ratio adapters stock even. That make it easy to change. And any good speedo shop can get you one.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Take your drive gears, and hold them with either the long shaft end towards you, (cable end) or opposite. You will be looking at the "edge" of the gear. Look closely on that little flat edge before the teeth start. I have about 8 Th400 gears, and all but one or two have the tooth count "cast" into either of those edges. You just gotta know where to look.
     
  12. Brandy,
    I've got a 89 K5 blazer with 3.73's from the factory and I had 33" tires on it but just put 35"s on it a couple weeks ago. I have a 700R4 trans, and I'm assuming you do also. The stock driven gear in the transfer case in mine was a 38 tooth, and with 33's my speedo was about 4 mph slow at 60 mph. If this is like your setup, then get a 36 tooth speedo gear for $4.50 from GM. I had to go to a 34 tooth speedo gear to account for the 35's on my K5. It is now only 1 mph slow at 60 mph and that's close enough for me. Let me know if you have any other questions.

    Later,
    Doug
     
  13. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    That's a tranny gear drive. I think the t-case gear drives are different....could be wrong though (wouldn't be a first).
     
  14. dyeager535

    dyeager535 1 ton status Premium Member

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    I took some out of a 208, so I can take a look at them...something like 3 gears though, but since the t-case was shattered, wasn't sure what came from where. I'll have to take them out, but all the gears looked similar, if not the same, to the gears used in the TH350's, Th400's, 200's, 200-4R's, and 700's.
     
  15. MudNurI

    MudNurI 1/2 ton status

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    I don't have the 700R4- I have a manual transmission. My speedo isn't slow- I'm fast. i believe I probably have either a 19 or 21 tooth..but am not sure yet, as I didn't get it off yet. my 3.73's didn't come from the factory...i'm trying to clean off my sticker in my glove box so I can read it....I'll let you know what I find
    Brandy
     
  16. Blue85

    Blue85 Troll Premium Member

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    The tranny and T-case speedo gears are the same, at least for many years. So at least something is right with the world. To make matters even better, the number of rotations of the speedo cable per mile is the same for almost all vehicles (at least domestic) for decades!

    First, get the chart of drive and driven gears with the colors and tooth counts from the links above. Then take out your stock gear and shine a flashlight insdide the case to identify the color of the drive gear. Using this information, you can either use one of the charts to find the right combo or just compute percentages.

    You take the percentage change of your tires (i.e. 33"/29") and the percentage change of your gears (i.e. 3.08/3.73) and the two multiplied together gives you the percentage change needed in speedo gear. But make sure you use the actual loaded diameter of your tire and not the height listed on the sidewall. Your 33's are probably 31.5" or 32" tall. Then see if you can get to where you need to be with only a driven gear change, since it is really easy to change and the drive gear takes some work.

    Your other option is to clock some mile markers on your odometer and find out the percentage you are off by. You could also get some gears from a junk yard and trial and error them. Do some calculations to have a guess for a starting point and then add or take away a tooth from there. But it's really a better use of your time to just crunch a few numbers up front. Mine ended up spot-on the first time.
     
  17. Thunder

    Thunder 3/4 ton status

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    Here's a handy dandy speedo gear calculator and more info:(link) <a target="_blank" href=http://www.tciauto.com/tech_info/speedo_gears.htm>TCI speedo gear info</a>
     
  18. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Yellow pages usualy speedometer. Most reasonable sized towns will have a shop that does spedometer calibration. They usualy have rollers so they can see how far off the speedo is with the the tires. What that tire says on the side wall is not what it is actually going to measure. If they test it then they will be able to figure out what gear is the best to get an acurate reading. Most of those places if they have been around could probably tell you right off the top of their head what gear you need. They might even have the adaptor boxes and save pulling the tail shaft of the tranny off.
     

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