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Calculating speedometer gear info

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by X MONTE SS, Sep 26, 2002.

  1. X MONTE SS

    X MONTE SS Newbie

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    I have a 1985 k5 with turbo 700 and a switch from 3.08s to 4.11s im running 33" tires and the speedo is way off.
    Thanks everybody
     
  2. Blue85

    Blue85 Troll Premium Member

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    You'll have to find out what color your drive gear is. Just pull the speedo gear (driven) out and shine a flashlight in the hole. Then use this formula:
    # teeth in Driven Gear = (Drive Gear Teeth# X Axle Ratio X (20168 / Tire Diameter))/1001

    This formula works for Chevy, Ford and Chrysler because speedometer cable speeds are the same for all of them. That is a Ford speedo will read the same if swapped into a Chevy truck.

    Remember to use the true tire diameter and not it's rated diameter. Chances are good that you will be able to change the drive gear only and not the driven gear.

    The gears are the same whether they are in the transfer case (4x4) or in the tailshaft housing (2WD).

    Drive gear tooth count / color:
    15 - gray
    17 - red
    18 - blue
    19 - yellow
    20 - brown

    Driven Gear tooth count / color
    Truck Gears:
    18 - brown
    19 - dk aqua
    20 - silver
    21 - red
    22 - gold
    23 - maroon
    24 - dk blue

    Passenger:
    34 - lt green
    35 - orange / pink
    36 - white
    37 - red
    38 - blue
    39 - brown
    40 - black
    41 - yellow
    42 - green
    43 - purple
    44 - dk gray
    45 - lt blue
     
  3. Blue85

    Blue85 Troll Premium Member

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    BTW: I am not sure about the breakdown between "passenger" and "truck" above. I don't know who uses the low tooth count gears listed under "trucks". I am running a 44 tooth dark gray driven gear on a 18-tooth blue drive gear in my NP208 case with 4.10 gears and 35" tires (really ~34").

    You can also do it all in ratios: (4.10/3.08)*(stock tire height/33 tire height)*number of teeth in existing gear.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    The difference *might* be the size of the driven gear "sleeve" and gear itself. (the piece that holds the driven gear in place) I know TH350's (or was it TH400's?) have two different "sleeve" sizes, and also that the different tooth counts for driven gears require different sleeves.

    I guess the reason for different sleeves is the gear "spacing", which is also why rebuild manuals state (some anyways) not to rotate the sleeve as you install it, as the driven gear is not perfectly centered in the sleeve.

    FWIW, just like all GM ring gears, all GM drive and driven gears I have seen have the tooth count stamped on the edge. (just look close) Some people don't trust the color method, although I haven't seen any discolor enough to make ID by color impossible.
     
  5. Bubba Ray Boudreaux

    Bubba Ray Boudreaux 1 ton status

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    Okay..........this math stuff is way above me........

    Here's what I got right now. 35's and 3.73. Best guess the speedo is about 10 mph slower than actual speed in the 70mph neighborhood. Case has a green gear. Case that is going into the truck has a yellow gear. What do y'all think?
     
  6. BrianDamage

    BrianDamage 1/2 ton status

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    [ QUOTE ]
    Okay..........this math stuff is way above me........

    Here's what I got right now. 35's and 3.73. Best guess the speedo is about 10 mph slower than actual speed in the 70mph neighborhood. Case has a green gear. Case that is going into the truck has a yellow gear. What do y'all think?

    [/ QUOTE ]

    when you say the cae has a gear, are you meaning the drive gear or the driven gear?

    The DRIVE gear slides over the rear output shaft (at least on my NP208). It slides over it pretty much like a collar. The DRIVEN gear is a cog-like gear that comes into the tailshaft section from the side, and the spedometer cable inserts into it.

    Also DO NOT go by the printed size on your tires, get a tape and measure the diameter from the ground to the top of the tire. My 33's only measured 31".

    You can goto this website and enter the numbers for the drive gear, rear gear ratio and tire measurement and it will calculate it for you. Then, just pick the driven gear that most closely matches that number.

    http://www.tciauto.com/tech_info/speedo_gears.htm
     
  7. Bubba Ray Boudreaux

    Bubba Ray Boudreaux 1 ton status

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    I was talking about the small driven gear.
     
  8. Triaged

    Triaged 1/2 ton status

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    All I have to say is Steve Chin is THE MAN!!!
    This is something I cut and pasted from a post of his on the 1st gen board.

    [ QUOTE ]

    The correct way for a speedo shop to calibrate a speedo is to ensure that the odometer reads correct at 1000 revolutions per mile, then make the speedometer head agree with that by either moving an adjuster in the speedometer head or by changing the magnetism of the flying magnets in the head.

    That said, you first need to figure out what gears you have in your transfer case (you already know your tire OD and final drive ratio). To calculate what gears you need, you first need to know the Speedometer Gear Ratio (SGR).

    Calculate that by:

    FDR = Final Drive Ratio
    TOD = Tire Outside Diameter

    SGR = 63360*FDR/(TOD*3141.597)

    In this case, with 38" tires and a 4.10:1 final drive, the SGR = 2.176030384

    Next, you need to know the tooth counts on the drive gear and driven gear. The only way to find out is to remove them and count.

    If you divide the number of teeth on the driven (output) gear by the number of teeth on the drive gear (the gear on the output shaft) and that result does not agree somewhat closely with the SGR, you need a ratio adapter (or you need to change one or both of the gears).

    In order to determine the ratio of the drive adapter you need, you simply:

    SOG = Speedometer Output (driven) Gear
    SIG = Speedometer Input Gear
    SGR = Speedometer Gear Ratio

    Adapter Ratio = SOG/SIG/SGR

    Note that some adapter ratios might be overdrive (greater than one) and some might be underdrive (less than one).

    Clear as mud?


    [/ QUOTE ]
     

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