vette61

I have noticed alot of guys on the forum switching out their posi's which I did some time ago (4:11 to 3:08). No doubt now that their speedometer readings are affected. I have been to those websites that have formulas for figuring out which driven gear goes with which drive gear but most say you need to know which drive gear you have before determining which driven gear you need.....everyone still with me Ok, so the question is....how do you tell? Unless you pull the tranny and then the tailshaft is there another way to figure it out? Did early BW T-10's only have one drive gear for example 8 teeth? All I know now is that anything above 60 mph the speedo reads 45 . I have tried a few different driven gears (more teeth/less teeth) and haven't found the right combo yet. This is driving me nuts...any advise on what to try next would really be appreciated.....Mike

vetrod62

You should be able to see the drive gear teeth thru the speedo cable hole in the tail shaft. Mark one of the teeth and count the number of teeth there are on one revolution of the tail shaft. Just like counting the teeth on a differental ring gear.
vetrod62

DZAUTO

Most, but not all, trannys have the 8 tooth drive gear on the mainshaft inside the tail housing. There were also 2 sizes of drive gears: 1.76in and 1.84in diameter. The driven gears were, and still are, available in 17-25 teeth from GM. Within these 10 different gears, there are also 2 sizes. The smaller size (17-22 teeth) is used with the larger 1.84 drive gear and the larger size (22-25 teeth) is used with the 1.76 drive gear. If you use a large driven gear with the large drive (1.84) gear, the large driven gear will eventually get chewed up. If you use a small driven gear with small (1.76) drive gear, it will either very minimally engage or not engage at all. If I were going to roll the dice, I'd bet that there is a much greater chance that your tranny has the larger 1.84 drive gear (most trannys did have the larger one). Therefore, the corrosponding driven gears that will work in your tranny are the smaller, 17-22 teeth gears. ALLLLLLLLLLLLLLLLLLLLLLLLL the driven gears are color coded and the large and small 22 teeth driven gears are also different colors (the big one is green and the small one is gray, sometimes called silver).
GENERALLY speaking, a change in one tooth (up or down) of the driven gear will result in about a 5mph change in the speedo. Also, GENERALLY speaking, for each change in rear gear ratio, such as going from 4.11 to 3.70 or to 3.55 or to 3.36, etc, will require changing the driven gear by one tooth. Thus, if your car had a 4.11 rear gear and you switched it to a 3.08, then you would have changed the rear by 4 ratios, and probably a change to 4 FEWER teeth of the driven gear would get you in the ball park, if not right on the money. So, if your driven gear had 22 teeth with a 4.11, then a change to an 18 teeth driven gear should get you close.
Hope this helps.

JohnZ

The 1.76" dia. DRIVE gear (3708145) is .531" wide, and the 1.84" dia. DRIVE gear (3708144) is .610" wide (both are 8 teeth); you can check the width through the speedo adapter opening with a cardboard gage.

bweaver999

So if the speedometer is 5MPH to fast, adding a tooth would drop it 5MPH? Just trying to verify my logic, seems adding teeth would reduce the speedometer.

JohnZ

Yes, adding teeth on the DRIVEN gear will reduce the speedo reading (cable runs slower); a one-tooth change results in about a 5% change in the reading.

GCD1962

I was told that the driven gears (the nylon ones) are no longer being made by Delco and stock is minimal right now. Suggest you buy which ones you need now

DZAUTO

OK, here are the GM part numbers/colors/teeth. I'll post them and YOU call your friendly neighborhood GM parts dept to check to see if the numbers are still good.
OR you can pull them up on gmpartsdirect.com

SMALL GEARS
3987917-17 teeth-purple (still good)
3987918-18 teeth-brown (still good)
3987919-19 teeth-natural (still good)
3987920-20 teeth-blue (still good)
3987921-21 teeth-red (still good)
3987922-22 teeth-gray (or silver) (still good)

BIG GEARS
3860345-22 teeth-green (still good)
3860346-23 teeth-black (still good)
3860347-24 teeth-yellow (still good)
3860348-25 teeth-orange (still good)

OK, OK, I went ahead and did your work for your.

CaveCreekChump

Thanks DZ,

You make me proud to be an Okie!

GCD1962

Good work - get em now while supply still exists

CaveCreekChump

GMPartsDirect.com did not recognize the part numbers for the driven gears. However, I found RiversideGear.com which seems to stock the correct stuff with their own part numbers. Their speedo gear catalogue is labelled "muncie-4-speed/speedometer-gears-drive-and-driven".

Hope these are the same for other GM transmissions; my new driven gear is going into a TH-400 in a '67 GTO.

DZAUTO

HUH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
That's where I just went to check the numbers and each one showed to be available! STRANGE!

DZAUTO

OK, I just went to gmpartsdirect and ordered 5 different gears by the part numbers that I posted (I like to keep at least 2 of each gear in stock at all times, 3 of some gears). The order was processed just fine and I was given a confirmation number.

CaveCreekChump

It is entirely possible that I am defective in some way. In fact, the spousal unit has been harping on this fact for decades.

CaveCreekChump

Problem solved! GMpartsdirect.com works like a champ.

My mistake was adding the number after DZs dash (i.e. -17 teeth)

Duh!

vette61

This morning I pulled the driven gear and counted the teeth....22 and it's brown so I'm not sure how it got in there but I'll bet bubba knows. Couldn't really see what drive gear I had since the tranny brace was in place. As suggested, I will try an 18 tooth........thanks for everyones inputs.....Mike