I finally found the driven gear housing I need for my TH350.

Which gears do I need for 2.73s gear with 255/50/16'' tires? The shop gave me a couple gears also. The one for the shaft is orange/tan color and the small gear is teal. Any idea what those are for?

I searched this site and the internet but I never got a clear answer as to what color is for what.

I also have a red gear from my 700r4 on for the shaft. And I have a purple shaft gear that came on the transmission. Will a combo of my small teal gear and any of the 3 shaft gears get me close with 2.73s?

 

Ok I finally found a site that explained everything better.

It said if your keeping your current rear-end gear and tire size that I should: Use the shaft gear from my 700r4. Then count the other gear (700r4- 37 teeth) and find the th350 gear that is half that. I have a teal color gear which has 19 teeth. Do you think this will work out correctly?

 

Ok, I'll try to answer clearly so that there is no confusion.

Your driven/drive gear ratio needs to be 2.11 (based on your tire size and rear end gearing). So if you have, let's say, a 17-tooth drive gear, you should have a 36-tooth driven gear.

Gear color can be misleading as it is not kept up, some aftermarket manufacturers do not follow it correctly. So count the teeth, that will give you the best possible information.

I am not 100% positive but I have this nagging feeling in the back of my brain that the TH350 and 700R4 drive gears (on the tailshaft) are not interchangeable. But I could be wrong, please someone correct me if I am.

Cont the teeth and let us know what gears you got from the 700R4 and TH350 and we can go from there.

Hope this helps.
Lou

P.S.: And to answer your question: the 37/19 combo won't be precise for your application, the speedometer would be off by 8% (e.g. showing 64 at 70mph).

 

The drive gear fit perfectly on the th350 shaft. Same diameter and mounting. But I have no idea about anything else.

The driven gear I have (the small one) is 19 teeth. They also gave me the drive gear that has 6 teeth. And my transmission came with a purple gear that has 6 teeth also....but it is a hair wider.

How do I actually calcuate it and see what gears I actually need?

Also does anyone know what tooth a stock 700r4 red gear is? That is whats on my th350 now. For some reason I was under the impression my stock gears were going to work so I put them on there.

I really wish I knew how to actually calculate what gears I need.

 

I heard that the 700 and the 350 gears would physically fit, but due to different tailshafts/driven gears, they needed different drive gears. Simply put, it'll fit, but won't work.

....my red gear that came from the factory on my 700 had 17 teeth.

Check out this site:
http://www.transmissioncenter.net/sp...n_______va.htm

Got a lot of info, AND bought my gears here.

 

* Rear Tire Diameter (Inches) = 26.00 (25.75)
* Differential Gear Ratio = 2.73
* Drive Gear Teeth Number = 10

Computation Results:
# Computed Driven Gear is 21
# Ratio of Driven Over Drive Gear is 2.1

So a 10 gear/21 gear works......OR:

* Rear Tire Diameter (Inches) = 26.00
* Differential Gear Ratio = 2.73
* Driven Gear Teeth Number = 19

Computation Results:
# Computed Drive Gear is 9
# Ratio of Driven Over Drive Gear is 2.1

.....a 9/19 works too.

 

...and in case you missed it with all the numbers flying around, you can't use a 700r4 drive OR driven gear.

The tailshaft on the TH350 puts the driven gear closer to the tailshaft so both gears have to be physically smaller to mesh together. That's why the 700r4 drive gear fits the tailshaft, but won't work. The larger driven gear needed to attain the proper ratio wouldn't even begin to fit/mesh with the larger drive gear.

(this is how it was explained to me. It makes sense though!)

 

Ok I need to find a 9 tooth drive gear then. Since I have a 19 tooth driven gear. Wonder how long it will take to get one in from a GM dealer.

I bet the gear the shop gave me is for a 700r4. Because the one that came with the trans is a little bigger around.

Out of curiosity...with the 19 tooth driven gear and the 6 tooth drive gear what ratio is that for?

BTW what is the formula for this?

::edit::: BTW thanks for the help. I appreciate it.

 

The formula is that your driven gear has to have 2.11x more teeth than the drive gear. Very simple.

So if you have a 9-tooth drive gear on the shaft, you need a 19-tooth driven gear. (or backwards, as you already calculated it)

The whole formula goes like this:

DRIVEN_TEETH = 20.15*REAR_RATIO*DRIVE_TEETH / TIRE_DIAMETER

TIRE_DIAMETER = WIDTH*2*RATIO/100/25.4 + RIM_SIZE

So for your 255/50/16 tires, it goes like this:
TIRE_DIAMETER = 255*2*50/100/25.4 + 16 = 255*100/100/25.4 + 16 = 255/25.4 + 16 = cca 26"

DRIVEN_TEETH = 20.15 * 2.73 * DRIVE_TEETH / 26 = 2.11 * DRIVE_TEETH

There you go.

 

Ok if I am trying to find the ratio for 3.42s I substitute 3.42 obviously. Where I am confused is here DRIVEN_TEETH = 20.15 * 2.73 * DRIVE_TEETH / 26 = 2.11 * DRIVE_TEETH

Do I take the total which is 15.903 (with my 6 tooth drive gear). Do I divide that number by 2.11 or 6?

I was just trying to see if I even have a chance of reusing my 6 tooth gear.

I have been all the way through business calculus lol...and I just want to make sure I am getting this formula correct so I know what gear I need to get.

 

From your tire size and rear end ratio, you calculate the ratio for your speedometer gears and use that to figure out what gearing you need.

For your current 2.73 gears, the speedometer driven/drive gear ratio is 2.11.
Meaning that if you have the 19-tooth driven gear, you'll need a 9-tooth drive gear.

If you change your rear gears to 3.42's, the driven/drive gear ratio will change to 2.64. So with a 9-tooth drive gear, you'd need a 24-tooth driven gear.

With a 6-tooth drive gear, multiply it by 2.11 for the 2.73's and you'll get 12.66, i.e. 13-tooth driven gear (not sure if they even make these).
Or multiply the 6 teeth by 2.64 for the 3.42's and you get close to 16.

Hope this makes sense.
Lou

 

...or you can use the calculator on the link I posted, or on just about every other transmission website!

 

It tells you everything about the 700r4 on that site. Just not that much about th350s. I am probably going to order some gears from that site soon.

 

A ratio is a ratio. As long as you know your rear gear (2.73) and your tire height, (26") you can find the right gears with the calculator.