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please help me np205 speedo gears

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by blazen 1971, Nov 18, 2002.

  1. blazen 1971

    blazen 1971 Newbie

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    hi,

    I have a 1971 with the np205 and 3.73's and 32" tires.

    Can someone please tell me what gear i need in the t-case to get my speedometer to read the proper speed. (right now it reads low). also please let me know where to get it. Thanks

    Tom
     
  2. bdkmiller

    bdkmiller Registered Member

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    I had to deal with this issue recently but I found that the difference was so minimal as to not be worth the effort. I'm going off memory here so I might not have it totally right-
    If it is set up for 31's & you go up to 33's then your speedometer will be off by 6% or so. Do I have this correct? There is an equation for this & maybe someone who remembers it can post it. But any speedometer shourl be able to hook you up.
    Good luck,
    Brian
     
  3. RingMaster4x4

    RingMaster4x4 1/2 ton status Premium Member

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    Go to 4lo.com and there is a calculator there that will tell how far off the seedometer is.
     
  4. swamp donkey

    swamp donkey 1/2 ton status

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    This is usless help, but cool. I was with my budy in his lifted truck the other day. We took out his new GPS unit to play with it and it told us how fast we were going, to the tenth of a mile. His speedo was off by about 8%
    If you can borrow a GPS, then you could tell for sure how close your speedo was. Then you could work backward from that and figure out exactly what gear you need.
    Just remember, speedo's aren't perfect even from the factory.
     
  5. Steve_Chin

    Steve_Chin 1/2 ton status

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    Speedometers and odometers are calibrated for 10,000 revolutions in a mile. You need to calibrate the gears in the transfer case to obtain a gear ratio of 2.35:1 to get the correct odometer reading, then calibrate the speedometer head to jive with that. Most likely, you will not be able to achieve the exact gear ratio and you may need to go with something that's somewhat out of whack and use a drive ratio adapter (I have a 20.2% overdrive adapter on mine that was installed by the factory because the factory set my TC up for 3.73s and 30" tires).

    FWIW, this is the formula for speedometer gear ratio: (63360 * Axle Ratio) / (3141.6 * Tire OD in inches) = SGR.

    Take the SGR and multiply it by the number of teeth on the drive gear to obtain how many teeth you need to have on the driven (output) gear: SGR * DGT = OGT
     
  6. Triaged

    Triaged 1/2 ton status

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    Steve, you da man!
    Where do you get all this cool info?
     
  7. Steve_Chin

    Steve_Chin 1/2 ton status

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    Uhhh...

    Gee...

    I dunno where I got it. It was just in the back of my head. Too many years of hot rods and race cars, I guess. LOL!
     
  8. POFF

    POFF 1/2 ton status

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    Most speedo shops will dyno it and make a gear box that goes between the T-case and speedo cable. I think they run about $75. The speedo gear in the T-case is not like the tranny gears. It is a large metal gear, I don't know if they come in different tooth counts.
     
  9. 69K5

    69K5 1/2 ton status

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    how do you find a speedo shop? theres nothing in the phonebook.

    thanks

    nathan
     
  10. Steve_Chin

    Steve_Chin 1/2 ton status

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    Most phone books list 'em in the Yellow Pages under "Automobile, Repairing & Service." Unfortunately, you need to wade through all of the other service shops.

    Another thought is to talk to a local speed shop or 4x4 shop. They usually have knowledge of a good speedo shop. Some alignment & chassis specialty shops also do speedo work.
     
  11. 69K5

    69K5 1/2 ton status

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    i'll have to look and see. there arent any that list it specifically. there is a specioalty 4x4 shop that can probably tell me. what is a good price? the specialty shop i speak of is crooks!!!!!!!!!!! and it violates all my principles just to call em but i will. i get tired of adding 14 to what the gauge says.

    thanks

    nathan
     

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