CUCV NP208 Speedo gear colors\tooth count

[weaponoffreedom]

My speedo was work great, and then one day it started to bounce when I got up to maybe 45 MPH or so.

I checked the cable,
I checked the connections

The speedometer does work, so I think the issue is in the gear set inside the tailshaft of the NP208. I have found a site that still sells the whole kit, but they are different colors and different tooth counts....

Does anyone know which is correct for an M1008?

I would like to have the parts on hand before I start the project if possible

 

[Sharecropper]

The OEM NP208 in M1008 came with a gray 15-tooth GM14037997 Drive gear inside the tailpiece and a yellow 41-tooth GM1362195 Driven gear in the sleeve that screws into the tailpiece. See attached PDF from the 34-P Technical Manual. If your speedometer is bouncing, IMO it is the Driven gear and the sleeve which is the culprit. I recommend you remove, inspect, and grease the gear & sleeve. That usually eliminates a bouncing speedometer needle, but if the teeth are worn, you should replace the gear. If you have changed tire/wheel size, you will need to use a different Driven gear for correct speedometer output. I covered how to calculate which gear in my thread - https://www.steelsoldiers.com/threads/np208-transfer-case-factory-drive-gear-id.184790/

Hope this helps.

 

[Sharecropper]

And as a follow-up for those folks with larger tires who may not be mathematically endowed to calculate which Driven gear they need, I will add that fewer teeth on the Driven gear will speed-up the speedometer reading, while more teeth on the gear will slow down the speedometer reading. For every tooth on the Driven gear a difference of approximately 2 MPH of speedometer reading can be expected.

 

[Sharecropper]

And still more information - this link will provide the various GM gear part numbers, their colors and tooth counts http://www.73-87.com/7387garage/drivetrain/speedo.htm . Note there are two different sleeves used, GM25512340 for Driven gears with tooth counts of 34 through 39, and GM25512339 for Driven gears with tooth counts of 40 through 45. The M1008, M1028, M1010 etc. trucks have OEM yellow 41 tooth Driven gears and therefore have GM25512339 sleeves due to the 4.56 gear ratio. For the M1009 Blazers, they came with the same Drive and Driven gears as the trucks but used a special adapter which screwed onto the same sleeve which changed the output due to the M1009's higher axle ratio. The speedometer cable simply attached to this adapter.

 

[weaponoffreedom]

The OEM NP208 in M1008 came with a gray 15-tooth GM14037997 Drive gear inside the tailpiece and a yellow 41-tooth GM1362195 Driven gear in the adapter that screws into the tailpiece. See attached PDF from the 34-P Technical Manual. If your speedometer is bouncing, IMO it is the Driven gear and the adapter which is the culprit. I recommend you remove, inspect, and grease the gear & adapter. That usually eliminates a bouncing speedometer needle, but if the teeth are worn, you should replace the gear. If you have changed tire/wheel size, you will need to use a different Driven gear for correct speedometer output. I covered how to calculate which gear in my thread - https://www.steelsoldiers.com/threads/np208-transfer-case-factory-drive-gear-id.184790/

Hope this helps.

Thanks fella. I will do as you advise. Will report back.

I apologize to all if there is a thread on this already. I did a quick search and could not find your specific info....

 

[Sharecropper]

Thanks fella. I will do as you advise. Will report back.

I apologize to all if there is a thread on this already. I did a quick search and could not find your specific info....

If you have changed tire/wheel size, let me know the diameter in the center of the wheel assembly and I will be happy to calculate the correct Driven gear for you.

 

[weaponoffreedom]

If you have changed tire/wheel size, let me know the diameter in the center of the wheel assembly and I will be happy to calculate the correct Driven gear for you.

As of now, the tires are in good shape and are the factory 235/85/16, but when I need new tires, I think I an going up to 255/85/16. Thanks for the offer. I will look into the links you gave and report back.

 

[weaponoffreedom]

If you have changed tire/wheel size, let me know the diameter in the center of the wheel assembly and I will be happy to calculate the correct Driven gear for you.

Judging by the equation given in the link you supplied, the proper driven gear would be 45 tooth as opposed to 41 tooth.........any thoughts? 30.5 inch tires "factory size"

 

[Sharecropper]

Judging by the equation given in the link you supplied, the proper driven gear would be 45 tooth as opposed to 41 tooth.........any thoughts? 30.5 inch tires "factory size"

View attachment 954960

My calculations are the same as yours that a 45-tooth Driven gear will be correct for 30.5" diameter tires on a stock M1008 truck. However to double-check this calculation, I will suggest that you compare your exact speedometer output versus the actual ground speed to determine the amount of speedometer error. Also, did you actually jack-up the vehicle and verify the exact diameter by measuring the circumference in the middle of the tire, and then calculating the exact diameter by using the formula Diameter = Circumference divided by 3.14159 ( Pi )? Or did you simply assume the diameter by measuring the height of the top of the tire from the ground with the weight on the vehicle? I will add that the correct diameter of a brand new 235/85/R16 tire when properly inflated is approximately 31.7 inches (but varies per tire manufacturer), which would require a 43-tooth Driven gear. So you can see how a worn tire with a 30.5" diameter could impact the Driven gear needed. The smaller the tire diameter, the higher tooth-count the Driven gear needs to be. Conversely, the larger the tire diameter, the lower tooth-count the Driven gear should be. Until the tire diameter calculation results are below the 40-tooth Driven gear requirement, and then the sleeve will also need to be changed for the lower-tooth Driven gears.

Of course, all of this will be moot if you plan on installing larger tires soon. A new 255/85/R16 tire should carry a diameter of 33.1", which calculates to requiring the OEM 41-tooth Driven gear. So if you indeed plan on increasing tire size soon, I would simply install the same OEM yellow 41-tooth Driven gear.

Having said all this, you may find that your OEM Driven gear is fine, just the sleeve needs cleaning and greasing.

Hope this helps.

 

[weaponoffreedom]

My calculations are the same as yours that a 45-tooth Driven gear will be correct for 30.5" diameter tires on a stock M1008 truck. However to double-check this calculation, I will suggest that you compare your exact speedometer output versus the actual ground speed to determine the amount of speedometer error. Also, did you actually jack-up the vehicle and verify the exact diameter by measuring the circumference in the middle of the tire, and then calculating the exact diameter by using the formula Diameter = Circumference divided by 3.14159 ( Pi )? Or did you simply assume the diameter by measuring the height of the top of the tire from the ground with the weight on the vehicle? I will add that the correct diameter of a brand new 235/85/R16 tire when properly inflated is approximately 31.7 inches (but varies per tire manufacturer), which would require a 43-tooth Driven gear. So you can see how a worn tire with a 30.5" diameter could impact the Driven gear needed. The smaller the tire diameter, the higher tooth-count the Driven gear needs to be. Conversely, the larger the tire diameter, the lower tooth-count the Driven gear should be. Until the tire diameter calculation results are below the 40-tooth Driven gear requirement, and then the sleeve will also need to be changed for the lower-tooth Driven gears.

Of course, all of this will be moot if you plan on installing larger tires soon. A new 255/85/R16 tire should carry a diameter of 33.1", which calculates to requiring the OEM 41-tooth Driven gear. So if you indeed plan on increasing tire size soon, I would simply install the same OEM yellow 41-tooth Driven gear.

Having said all this, you may find that your OEM Driven gear is fine, just the sleeve needs cleaning and greasing.

Hope this helps.

Thanks.

Yes very helpful, to your advisement, I am going to remove and inspect the assembly for defects, and adjust as needed. Is there a preferred grease that is over the counter?