NP208 Speedo Driven Gear Needed

[pgnavy]

Hey all, my speedometer is off by 4 MPH (too fast) at 60MPH, confirmed with GPS and a chase vehicle. I have stock rims (15x8) and tires 31x10.5. This is 87 V1500,700R4, NP208, w/GU6(3.42), 53K original miles; I bought this truck a few months ago after it sat mostly since '05, and I've done a bunch of work on it. It looks like PO had been in the tail shaft, you can see some black RTV squeezing out of the tail-shaft housing gasket(see transfer-case tag photo), maybe they screwed with the driven gear, who knows. Does anyone know what driven gear I need/teeth count? BTW, is that even the correct speedo housing for this setup: stamped 40-41-42-43? thx Paul ps. I know your gonna ask how many teeth the current gear has...i'll get that later for us.

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[pgnavy]

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As promised, an update.... I have a GREEN driven gear(42 tooth), and a BLUE Drive gear(18) in my NP208C (OE according to my GM parts book). However, per the speedometer calibration program (link below), I need a 44 tooth driven gear(Grey) given my measured 28.5 diameter tires/3.42 rear ratio. Once I replace with the (44) gear, I will let you know how it turns out when I road test later in the week.
Ideas welcomed, but I might have figured this out, we shall see.


I used this calculator to determine the driven gear size I need with "30.5 inch" Firestone (but actual measured 28.5 on the rims, on the ground) tires:
http://www.bgsoflex.com/speedo1.html
Bowling's Speedometer Calibration Program
Input Parameters Are the Following:
Speedometer Revolutions/mile = 1000
Rear Tire Diameter (Inches) = 28.5 (actual measured)
Differential Gear Ratio = 3.42
Drive Gear Teeth Number = 18
Computation Results:

• Computed Driven Gear is 44

• Ratio of Driven Over Drive Gear is 2.4

This is the reference document for the 700R4, however, the drive gear in the NP208C is in it's tail-shaft and looks different, but the color codes are the same.

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[HotRodPC]

Yep, you're OK to go with the "DRIVEN" gear of a 700r4/Th350, but as you can see inside the shaft on your Tcase, the same "DRIVE" gear will not work. It does appear you have it figured out to replace the "DRIVEN" gear that is compatible with your current "DRIVE" gear.

Do let us know how it comes out in the end. Keep in mind, your speedo may not be dead nuts but it's going to be close enough if not dead nuts, that you should be able to live with it. You can get some dead nuts but in most cases if you're within 5% of accuracy either direction that's the best you can expect unless you want to either up your tire size a hair or down size a hair.

 

[pgnavy]

Good news - the new 44 tooth gear got me to 1MPH accuracy. I suppose the important learning is to measure the tire diameter while on the truck, on the ground. The difference in manufacture claimed diameter (30.5) was off by 2", (28.5).
What perplexes me is the GM parts book says I need 42 tooth driven, which as mentioned earlier, was originally installed for my factory setup (3.42, 31x10.5x15). Maybe my speedometer is just old and out of accuracy, or maybe the original tires 30 years ago measured 30.5", who knows, but it sure is good now with the 44 tooth driven gear. For me, the Bowling Calculator Program was better than the GM Parts Book.

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[HotRodPC]

I just read this last post and not the entire thread so if it hasn't been mentioned yet, Don't forget the purpose or a RADIAL tire. Radial tires, not Bias Ply, but Radial tires, whether they be steel belted or fibreglass belted which I don't even think exists anymore, is to grow taller and thinner with higher speed. This is why it can be very hard to have a speedo that is 100% accurate.

 

[idahovette]

Ok now I need help . Grandson's square lost the speedometer. New cable didn't do it. The driven gear square drive rounded out.. The catch is it is a steel gear (203 case) with 20 teeth. I have no idea how this happened. Anybody got anything like this left over?? Please check your stashes for me, he doesn't need a ticket

 

[LateOnTheBrakes]

Good news - the new 44 tooth gear got me to 1MPH accuracy. I suppose the important learning is to measure the tire diameter while on the truck, on the ground. The difference in manufacture claimed diameter (30.5) was off by 2", (28.5).
What perplexes me is the GM parts book says I need 42 tooth driven, which as mentioned earlier, was originally installed for my factory setup (3.42, 31x10.5x15). Maybe my speedometer is just old and out of accuracy, or maybe the original tires 30 years ago measured 30.5", who knows, but it sure is good now with the 44 tooth driven gear. For me, the Bowling Calculator Program was better than the GM Parts Book.

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Where can I find this book? I'm trying to figure out what speedo gears I need for my 3.08 rear gears.

 

[HotRodPC]

Where can I find this book? I'm trying to figure out what speedo gears I need for my 3.08 rear gears.

You don't need a book. You need the forumla to calculate your gear ratio and tire size. I had a link to a site that has the calculator and the colors of the gears according to tooth count. I'll see if I can't dig that up. I know it's not in my favorites anymore.

 

[HotRodPC]

Where can I find this book? I'm trying to figure out what speedo gears I need for my 3.08 rear gears.

This info should help out some. Not the sight I had in mind, but pretty close to same info. They also have the calculator there. Type in your tire and gear ratio, and either of the toothed gears you have, the drive or driven gear, and it should tell you which other gear you'll need. https://www.tciauto.com/speedometer-gear-calculator

 

[LateOnTheBrakes]

Yeah, I'm familiar with the site. Just trying to figure out which color my truck should have had from the factory to decide if the transfer case is out of the V6 or diesel.

 

[HotRodPC]

Yeah, I'm familiar with the site. Just trying to figure out which color my truck should have had from the factory to decide if the transfer case is out of the V6 or diesel.

I don't know that powertrain makes a difference. The color are all decided by the # of teeth and the ratios to the axles and tire size.

 

[LateOnTheBrakes]

Right. The CUCV would have had 4.56 gears. The V6 had 3.73 gears.

 

[LateOnTheBrakes]

15, 40.

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[LateOnTheBrakes]

Assuming the np208 has the same gears as the 700R4 I cannot get the speedometer correct without changing the drive gear. Even then I would need a new housing and the smallest driven gear.

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[LateOnTheBrakes]

I need to replace the slip yoke and seal anyway at the output shaft. How much effort is it to remove the rear housing? Any internal snap rings or anything that prevents it from sliding off? I know the drive gear has a little snap ring on it.