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How can I determine the speedometer drive gear style and retaining clip required(if any) for a 350 TH transmission? I removed the 6" tail housing and found out that the drive gear was missing. Also missing was the driven gear in the tailshaft gear housing. I am unsure of the age of the tranny if that even matters. I am in the process of installing a Dakota Digital speedo. Any help would be appreciated.
 

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Mr Bob said:
How can I determine the speedometer drive gear style and retaining clip required(if any) for a 350 TH transmission? I removed the 6" tail housing and found out that the drive gear was missing. Also missing was the driven gear in the tailshaft gear housing. I am unsure of the age of the tranny if that even matters. I am in the process of installing a Dakota Digital speedo. Any help would be appreciated.

the style & size of the gears will depend on the tail housing design you have.

Chevy is a different design from the Buick , Olds. Pontiac tail housings and use different gears.

A non chain store trans shop would be more helpful usually for parts & information.
 

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Mr Bob said:
How can I determine the speedometer drive gear style and retaining clip required(if any) for a 350 TH transmission? I removed the 6" tail housing and found out that the drive gear was missing. Also missing was the driven gear in the tailshaft gear housing. I am unsure of the age of the tranny if that even matters. I am in the process of installing a Dakota Digital speedo. Any help would be appreciated.
in the extension housing.. how big is the hole ? if its about 1" I'd toss a 10 tooth drive gear and 16 tooth driven, and try it. if its the 2 1/2" hole, then you need something like an 18T drive and 37T driven.

I'd suggest you just take it to a tranny shop who has a whole drive/driven
gear assortment kit, so he can swap them out as needed to get your speedo registering correctly.

Its hit and miss, dude. depends on axle ratio, tire size, etc etc
 

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It can be hit and miss. But someone out there has a formula to plug the numbers into and get you very close. I used to have to calculate when I worked for a freightliner dealer, but it was different since it was digital data........Brian
 

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Forgot to add they also show the clip and retainer for the 350 housing. If you are going electric you should be able to set the proper speed in the speedometer no matter what gears you are running. Best to call for sure. 1-888-877-1008 hope this helps.
 

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If you don't mind doing a little math, go to http://novaresource.org/speedo.htm and you'll find a formula and a chart. You'll need your rear end ratio and tire diameter. Pick whatever gear set that matches the ratio from the formula, and head down to the GM dealer for the parts. Don't forget to get the clip, if needed.

http://tciauto.com/Products/TechInfo/speedo_gears.asp has a tire diameter calculator, and you can order gears from them, too.
 

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TransLab said:
in the extension housing.. how big is the hole ? if its about 1" I'd toss a 10 tooth drive gear and 16 tooth driven, and try it. if its the 2 1/2" hole, then you need something like an 18T drive and 37T driven.

I'd suggest you just take it to a tranny shop who has a whole drive/driven
gear assortment kit, so he can swap them out as needed to get your speedo registering correctly.

Its hit and miss, dude. depends on axle ratio, tire size, etc etc

He's right. The drive gear is inside the transmission on the tailshaft, the driven gear is on the end of the cable. This will be really handy information when seeking parts.
 

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Discussion Starter · #11 ·
TH 350 Speedo

Thanks for all the information which enabled me to calculate the drive and driven gears needed. The local GM parts department was a bust. Seems like they don't want to be bothered without a part number.I couldn't take the tranny to a local transmission shop since it was installed in the car and the car was out of commission. That said, I contacted P.A.T.C Speed Shop and ordered what I needed. They were very knowledgeable & helpful and didn't need part numbers. I now need to know how tight the 4 bolts attaching the tailshaft to the tranny should be. They were real tight when I removed it.I have the same question for attaching the drive shaft u joint to the rear end. (I don't have any of the torque specs.)
 

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As far as the torque needed on the tail shaft not sure but the fellow at PACT will tell you not to over torque those bearings on the u joints -17 recommended. I am sure they can tell you the tail shaft torque. I didn't torque the tail shaft. I just tighten them up good and snug ( be sure and use the X pattern and tighten the bolts up a little at a time if you know what I mean). Reason I'm familiar with the torque on the U joints is that about two weeks ago I was doing the same thing you are going through- I also ordered a gear and retainer plate from PACT. Good luck with your project.
 
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