Changing tranny/speedo gears [Archive] - Team Camaro Tech

: Changing tranny/speedo gears


Strick
Sep 26th, 07, 08:02 AM
I have searched and read many threads regarding which drive/driven gear I need to get my new tranny/rear end combo calibrated (TH350/27"tire/3.70). I think I need a 20 / 7 combination, but how do you install them? Are the behind the cable receptor on the tranny? I have not even looked at the tranny, but I now the car has the exhaust system on it and driveable, I would like to know how fast I am really going.

Thanks!

click
Sep 26th, 07, 08:16 AM
You will need to pull the tail section off the tranny to get at the internal 'drive' gear teeth to change it, then the 'driven' gears do change out behind the speedo cable connection.

tgifford5
Sep 26th, 07, 08:42 AM
You will need to pull the tail section off the tranny to get at the internal 'drive' gear teeth to change it, then the 'driven' gears do change out behind the speedo cable connection.

How much of a job is that? I want to do this also, but not until I change my rear end gears w/posi. Can the tranny still be installed in the vehicle to do this?

Strick
Sep 26th, 07, 08:45 AM
Click - what are the chances of just being able to change out the 'driven' gear, and not the drive. I just finished my car and got the X-pipe exhaust put on yesterday. I am not in a current state of mind to drop everything and pull off the tailshaft.

Thanks

KevinW
Sep 26th, 07, 08:50 AM
Strick and Ted, read this page thoroughly http://novaresource.org/speedo.htm. then compute your value and count the teeth on the drive gear (take speedo bullet out of trans, look through hole to mark a tooth and count, turn driveshaft with rear wheels up and trans in neutral). Then find the closest speedo gear (driven gear) from the charts. Buy correct color and tooth count on ebay (cheaper) and R&R driven gear in speedo bullet. No need to remove trans or tailshaft, it worked great on my car!

68zproject
Sep 26th, 07, 09:07 AM
I would try to just change out the driven gear. They are all the same except the turbo 400 ones. I have 3.73s and have the yellow 24 tooth gear (GM). I would try that and see where you are. Mine is pretty close, but I haven't checked it for being spot on. If that's not right you could try the 23 tooth black.
I don't know about autos, but on Muncies there is a bolt on the side of the speedo housing with a piece of flat metal that holds the "bullet" in the tranny. This bullet slides out once the bolt and holder are removed. Be careful as the gear can fall in the tranny. It goes in from the back side of the bullet and therefore comes out the back. Before you pull it out you might try to hold the end where the speedo cable goes into the gear with a needle nose or something.

click
Sep 26th, 07, 09:27 AM
As Kevin said, try the 'driven' gear change first and if that gets you close enuf to correct speed, then you can skip the 'drive' gear replacement. If you have to get into the 'drive' gear, its 4 bolts, remove driveshaft and lose a little fluid. Ive not had to remove mine, I just had the 'driven' replaced and it got me right on speed now. :)
http://www.carsbyjim.com/storage/Tranny/TH350trannyrearview.JPG

Strick
Oct 16th, 07, 08:30 AM
Is there a stock driven/drive gear setup that is the same in all TH350?

drdave69
Dec 6th, 07, 05:47 AM
This is great information since I am attempting to do this now with my Nova. I believe my Muncie is an M20 (stamped with a C suffix) but that doesn't seem to matter since the speedo output should be the same for all Muncies. It currently has a yellow 24 tooth driven speedo gear and the speed is way off. I tried an old blue 20 tooth gear and got nothing but a bouncy needle. The blue gear was smaller in diameter than the yellow and I was ignorant to the fact that they do not interchange.
I'll use the Novaresource guide to get me on the right path.

drdave69
Dec 10th, 07, 03:53 PM
I made a mistake on my prior post. My trans suffix is A, not C.
I removed the speedo bullet and driven gear. Counted the number of teeth on the drive gear and it appears to be 7 (it is yellow). My calculations put me at 2.67 ((3.55 X 20.2) / 26.85), but going by the chart at Novaresource, there isn't a match for me. My driven gear is about 7/8 in diameter instead of 3/4 or 1-1/2. What the heck do I have?

Strick
Jul 28th, 09, 09:07 PM
A thread back from the dead. I think almost 2 years is enough procrastinating.

To remove the driven gear, is it as simple as removing the 3/8" head bolt that secures the keeper and pull it out? Tranny = TH350

I am hoping it is as simple as that and a 6 mile trip to Summit for a 22 tooth grey TCI driven gear to replace what looks white to me from the outside of the tranny.

Thanks guys!

cwb90
Jul 29th, 09, 07:59 AM
Does the tranny fluid need drained to change out the speedo gear on a TH350?

Clint

BPOS
Jul 29th, 09, 08:54 AM
A thread back from the dead. I think almost 2 years is enough procrastinating.

To remove the driven gear, is it as simple as removing the 3/8" head bolt that secures the keeper and pull it out? Tranny = TH350

I am hoping it is as simple as that and a 6 mile trip to Summit for a 22 tooth grey TCI driven gear to replace what looks white to me from the outside of the tranny.

Thanks guys!

Yes! It's that simple.

click
Jul 29th, 09, 09:46 AM
Nope fluid is below that level, might get a dribble but no need to change or drain it.

JodysTransmissions
Jul 29th, 09, 09:53 AM
There were 4 speedo drive gears (7, 8, 9 , 10) for a TH350; orange, white, black, purple.

What color is yours? Just remove the speedo bullet to see your drive gear for the color.

69 z11
Jul 29th, 09, 10:54 AM
I recently did a rear end change, 3.07 to 3.73. My tire diameter is 25.2". I have a TH350. The driven gear in the tranny was natural (white) with 19 teeth and a diameter of 0.8". I calculated that I needed a 24 tooth gear. The yellow 24 tooth gear everyone sells is 0.95" in diameter. In fact, all TH350 driven gears over 22 teeth are 0.95" diameter. So, what do you do? Well, you can force it to fit, which I did, and the speedometer works great. The problem is, it may function OK for a while, maybe even for a long time, but because the gear is oversize, it is under pressure and it can break off or it can ruin the driven gear. I talked to some tranny guys (no pun intended) and they say the drive gear usually shatters and the bits and pieces end up in your pan. Anyway, I think the correct way to do this is to figure out how many teeth you need on the driven gear, then match the drive gear to it. All 0.8" driven gears should be paired with a 1.84" drive gear and 0.95" driven gears should be paired with a 1.76" driven gear. From the chart below, I need to order a black 1.76" drive gear and change it out before something breaks. That's what I'm going to try, will report back when I get it done.

Here are the gears and part numbers for reference:
(compiled from several sources, TCI has most of this)

TH350, Powerglide & Muncie 4 Speed Speedo Driven Gears
Part # Teeth Color Diameter Drive Gear Diameter
3987917 17 Purple 0.8" 1.84"
3987918 18 Brown 0.8" 1.84"
3987919 19 Natural 0.8" 1.84"
3987920 20 Blue 0.8" 1.84"
3987921 21 Red 0.8" 1.84"
3987922 22 Silver 0.8" 1.84"

3860341 18 Brown 0.95" 1.76"
3860342 19 Natural 0.95" 1.76"
3860343 20 Blue 0.95" 1.76"
3860344 21 Red 0.95" 1.76"
3860345 22 Green 0.95" 1.76"
3860346 23 Black 0.95" 1.76"
3860347 24 Yellow 0.95" 1.76"
3860348 25 Orange 0.95" 1.76"


TH350, Powerglide & Muncie 4 Speed Speedo Drive Gears
Part # Teeth Color Diameter Driven Diameter
6271537 7 Pink 1.84" 0.8"
6261783 8 Black 1.76" 0.95"
6261784 8 Orange 1.84" 0.8"
6261782 9 Green 1.84" 0.8"
6261785 10 Purple 1.84" 0.8"

Strick
Jul 29th, 09, 10:56 AM
Thanks for confirming Al - I appreciate it!

Jody - I have not pulled it, but looked at it last night. It looks white at the tip. I think I need to go with grey (22 tooth). 3.70 (9 inch) gears, 27" tire (255/60R15). However, I do not know what drive gear is in the end. The tranny is a 'new' rebuilt from Summit 450hp rated. I don't want to hassle checking. On my /vert I have to remove the driveshaft, crossbrace and exhaust (X-pipe to get at the tail). I would rather guess than hassle it. Thanks for your input!

- Oh, and I love the 'tranny guys' commentary!

Thanks!

JodysTransmissions
Jul 29th, 09, 05:22 PM
Jerry,

These speedo driven gears have been discountinued:

3860341 18 Brown 0.95" 1.76"
3860342 19 Natural 0.95" 1.76"
3860343 20 Blue 0.95" 1.76"
3860344 21 Red 0.95" 1.76"

JodysTransmissions
Jul 29th, 09, 05:35 PM
Thanks for confirming Al - I appreciate it!

Jody - I have not pulled it, but looked at it last night. It looks white at the tip. I think I need to go with grey (22 tooth). 3.70 (9 inch) gears, 27" tire (255/60R15). However, I do not know what drive gear is in the end. The tranny is a 'new' rebuilt from Summit 450hp rated. I don't want to hassle checking. On my /vert I have to remove the driveshaft, crossbrace and exhaust (X-pipe to get at the tail). I would rather guess than hassle it. Thanks for your input!

- Oh, and I love the 'tranny guys' commentary!

Thanks!

If you have an 8 tooth white drive gear, then you are correct needing a 22 tooth driven #3987922.

Your SGR (speedometer gear ratio) is: 2.75
So, whatever drive gear you have, you times 2.75 for the correct driven gear. 8 x 2.75 = 22 or 7 x 2.75 = 19.25 or 19 and so forth........

Jerry has brought up a great point about the correct drive diameter for the mating driven gear diameter for longevity. We see this speedo gear mis-match all the time.

Strick
Jul 29th, 09, 11:30 PM
Thanks again Jody!

Here is the update. I went to Summit after lunch and purchased the grey 22. After a small show and shine tonight I pulled the cable bullet (at least it looks like a large .38). The gear inside is a light shade of green, and had 20 teeth. Unless it was a blue gear stained light green from the tranny oil -?? I measured the diameter of both with digital calipers and the one coming out measured .78 and the one going in measured .80, so close enough.

The crappy thing was (is) the keeper bolt was put in stripped, but thats not all it was a 10mm head - on a GM tranny? Anyway, I put in the same bolt 10m head (5mm?). I was able to snug it. I will keep checking it.

Thanks again to all who responded!

cwb90
Jul 30th, 09, 09:22 AM
Strick,

Did you have to drain some tranny fluid to change the gear? I have a TH350.

Clint

Strick
Jul 30th, 09, 10:37 AM
Another update. The grey-22 tooth corrected about 14 of the 20mph that the speedo was off at 70mph (actual was around 50).

Now at 45mph (my wife's '08 Mercedes was the pace car) the speedo is reading 51 or 52mph in the Camaro.

I guess I will be buying the black 23 TCI gear today.

Strick,
Did you have to drain some tranny fluid to change the gear? I have a TH350.

About 20 ounces drained out from the cable/gear hole with the car about 8" off the ground in the front.

JodysTransmissions
Jul 30th, 09, 07:25 PM
By your numbers, it sounds as if you have a 9 tooth drive which is a 1.840" diameter gear. The 23 will not work with an 1.840" drive gear.

It might be best to bite the bullet and buy the correct matched speedo gear set for your transmission.

Strick
Jul 30th, 09, 08:21 PM
Or, I was sold 3.90's with my 'custom' 9" rear end. Although I requested 3.70's (this isn't my first rodeo with gearing!). I am going to take it around the block with the GPS to verify speed. Stay tuned!

Strick
Jul 30th, 09, 08:55 PM
o.k. just got back from my GPS run. I am now real close to 10% off 50mph speedo was 45.2-actual, 35 mph speedo was 32.2-actual, 25mph was 22.8.... I was doing this by myself so I have to drive and look at the GPS and speedo all at the same time. Given the fact that I am male that could account for at least 5% multi-tasking error. I am about done with this little exercise. I think bottom line is I was sold a higher (lower -tomato/tomatoe) ratio than I wanted.

Jody - I dont' know what you mean by the 23 not working. It wont' work, won't read, won't read accurate...............why will a 22 work. This IS my first speedo rodeo!

Thanks again!

JodysTransmissions
Jul 31st, 09, 05:47 PM
Jody - I dont' know what you mean by the 23 not working. It wont' work, won't read, won't read accurate...............why will a 22 work. This IS my first speedo rodeo! Thanks again!

You have a 1.840" diameter drive gear.

Typical Chevrolet TH350 Speedo gears:

17 thru 22 tooth small diameter speedo driven gears will ONLY work with the 7, 8, or 9 tooth 1.840" drive gear.

22 thru 25 tooth large diameter speedo driven gears ONLY work with the 8 tooth 1.760" drive gear.

mike p
Jul 31st, 09, 09:49 PM
It was already mentioned, but I also used this chart to select my proper driven gear: www.novaresource.org/speedo.htm

(rear axle x 20.2, divide that # by tire diameter, then check the chart for teeth combos)

My TH350 now has a 9 tooth green drive gear and a black 23 tooth driven gear.

JodysTransmissions
Aug 1st, 09, 04:13 AM
My TH350 now has a 9 tooth green drive gear and a black 23 tooth driven gear.

How long have you used this combination? You are using a large 1.840" drive gear and a large driven gear? Your speedo gear mesh is too tight using the 9 x 23 combo. Eventually your speedo needle will bounce and not read a steady mph.

A better choice would be the 7 x 18 or the 8 x 20 combo for longevity.

mike p
Aug 1st, 09, 04:52 PM
I've only had the car back from body shop and mechanics for a few weeks. Prior to all the restoration process, it still had the original PG transmission. The mechanic verified the drive gear in the freshly rebuilt TH350 transmission, and so I just followed the Nova chart and bought the driven gear from NPD. Thanks for the advice, I will keep an eye on the speedo and replace the drive/driven gears if I have any related problem. Also, I have a 3.36 axle gear and 26.1 " tire, so if you have any further advice, I will gladly take it.