Gear Vendors overdrive speedometer gear

[Enforced_Leo44]

Hi everyone,

My CUCV has a GV Overdrive unit installed, it has been by a previous owner, dating back probably 15 years at the least.

It works great and it really gives some relief to the engine at highway speeds.

Now, the problem I have is that the speedometer reads about 15% less than the real speed, say like when doing REAL 45mph, the speedo shows 40mph. Conclusion: either I recalibrate the speedometer (no thanks) or I get a new speedometer gear (yes please).

Does anybody know what kind of speedometer driven gear I need to get? Is it a proprietary design or is it something more "universal" like a TH400 gear?

Thanks in advance!
Leo

 

[chevymike]

I don't think it proprietary but I bought mine from GV as I also had to buy the NP208 adapter (mine was used with a 205 adapter). When I put it in, it seemed like it was a standard GM style.

 

[Enforced_Leo44]

I don't think it proprietary but I bought mine from GV as I also had to buy the NP208 adapter (mine was used with a 205 adapter). When I put it in, it seemed like it was a standard GM style.

I'll take off the speedo gear assembly on mine and check, thanks for the heads up!

 

[Sharecropper]

You might find this helpful -

https://www.steelsoldiers.com/threads/calibrating-speedometer.223246/

 

[Sharecropper]

You might find this helpful -

https://www.steelsoldiers.com/threads/calibrating-speedometer.223246/

As a follow-up, the OEM M1008 NP208 transfer case came from the factory with a 15-tooth gray drive gear, GM part #14037997, and a 41-tooth yellow driven gear, GM part #1362195, and a GM #25512339 sleeve. To make sure a previous owner hasn't changed these, you can easily remove the driven gear sleeve by removing the 10mm bolt which holds the retainer and sliding out the sleeve. Spray it down with cleaner beforehand to keep any crud from getting into the transfer case when the sleeve is removed. The sleeve has an O-ring to seal it into the hole, just grab the thing and twist it out. Then you can shine a flashlight into the hole and confirm the drive gear in the transfer case is indeed the OEM gray gear. To speed-up the speedometer reading, the tooth-count of the driven gear should be reduced. For a 5 mph speedometer indicated increase you will need to obtain a replacement driven gear. The lowest tooth-count driven gear that the OEM #25512339 sleeve will accommodate is the 40-tooth black gear. However, because each reduction in tooth-count will increase the speedometer approximately 2 mph, you will probably need to go to the 38-tooth blue or 39-tooth black driven gear, which will also require the #25512340 sleeve. The best way to definitively determine which drive gear you need is to go through the calculation process as described in the link to my thread above. All these parts are readily available on Ebay, just type in the GM part number in the search bar.

Hope this helps.

 

[Enforced_Leo44]

As a follow-up, the OEM M1008 NP208 transfer case came from the factory with a 15-tooth gray drive gear, GM part #14037997, and a 41-tooth yellow driven gear, GM part #1362195, and a GM #25512339 sleeve. To make sure a previous owner hasn't changed these, you can easily remove the driven gear sleeve by removing the 10mm bolt which holds the retainer and sliding out the sleeve. Spray it down with cleaner beforehand to keep any crud from getting into the transfer case when the sleeve is removed. The sleeve has an O-ring to seal it into the hole, just grab the thing and twist it out. Then you can shine a flashlight into the hole and confirm the drive gear in the transfer case is indeed the OEM gray gear. To speed-up the speedometer reading, the tooth-count of the driven gear should be reduced. For a 5 mph speedometer indicated increase you will need to obtain a replacement driven gear. The lowest tooth-count driven gear that the OEM #25512339 sleeve will accommodate is the 40-tooth black gear. However, because each reduction in tooth-count will increase the speedometer approximately 2 mph, you will probably need to go to the 38-tooth blue or 39-tooth black driven gear, which will also require the #25512340 sleeve. The best way to definitively determine which drive gear you need is to go through the calculation process as described in the link to my thread above. All these parts are readily available on Ebay, just type in the GM part number in the search bar.

Hope this helps.

My truck has a Np205, would the stock drive gear be the same? Hoping that the 205 came from a M1028

 

[Enforced_Leo44]

Also, thanks for all the info.

PS, since the speedometer gets the signal from the GV Overdrive unit, that would mean that I should look for the drive gear inside the OD unit, right? Do they use the same color coding as GM stuff?

 

[Sharecropper]

Also, thanks for all the info.

PS, since the speedometer gets the signal from the GV Overdrive unit, that would mean that I should look for the drive gear inside the OD unit, right? Do they use the same color coding as GM stuff?

You will need to do your own research regarding which drive gear & which driven gear you have. But the formula I provided stays the same, Determine your exact wheel diameter and then do the math. Then determine the tooth count of your drive gear, regardless of where it is located, and that will tell you what tooth count the driven gear should be for accurate speedometer display. If the speedometer cable attaches to the Gear Vendor unit, then reach out to GV and ask them what tooth count their unit is furnished with.