Just finished my rear swap and am now trying to sort out getting the speedometer correct. Using the TCI calculator, it's looking like I need a 24 tooth yellow driven gear (GM 3860347 - Part #) to work with my setup. Any idea on what housing I would need to go with this particular gear? My current one has "36 37 38 39" stamped on the outside of it. I'd like to get by without changing my drive gear if at all possible - I have no idea how to swap that piece out. Specs on the car: - 3:42 rear end - Green drive gear (I believe 9 tooth) - 25.33" tire diameter - TCI says I need a 24.48 tooth driven gear - TH350 transmission
I ran into this same problem a few years ago. There are 2 set ups for drive-driven gears for 350s and you have to make sure you are looking at the correct color/tooth combination. For many 350s, they use a bullet housing with a different size gear than the BOP 350s. If the housing the driven gear goes into is a large (about 2 inches diameter) where the speedo plugs in, you have the second type, which uses a different size set of gears. So a green BOP drive gear is different from a green chevy 350 drive gear. I had a 17-tooth drive gear (green) in a 72 BOP 350, which had a 37 tooth red driven gear, which was way off. I ordered a different housing (one that accommodates a larger number of teeth (I think up to 43 as the housing that was in the tran would only allow up to 39 if I remember right) and a I got a 42-tooth driven gear (also green). I think the tran came out of a car with 2.56 gears. Speedo was spot-on with GPS after this swap. However, a 3.43 rear may need more teeth than you can get in a driven gear, which would require you to replace the drive gear.
I went thru the same issue as you a few years ago, same rear gear (3.42) pretty close to the same tire diameter (26) I had to change BOTH the drive and driven gear, my speedo is now dead nuts on according to GPS If you can hold on for a day or two, I will go out and look to see what color/tooth count mine is for drive/driven is. Heres a few pics of BEFORE I switched gears.
Th400 here, 26 " tires, 3.42 gearing Pulled tailshaft housing replaced Drive gear with TCI 15 tooth Drive gear from Summit. Pulled speedo housing, replaced Driven gear with 39 tooth Driven gear. Spot on. If I change rear to 3.73s, all I gutta do is pull the speedo housing on the trans and swap Driven 39 tooth to a 42 tooth. Use the calculator on TCI website for your Th350 combo. I have a box of Driven and a few Drive gears for the Th400, not sure if the 350 uses the same plastic gears? Hit me up with a PM ifya want me to look thru my box of speedo gears....Jim/Rott
Thanks for the info guys. How hard was it to swap the driven gear? It looks like the tailhousing should just slide off with the driveshaft removed, but I'm a little hesitant to dive into pulling parts off the transmission...
Not hard. I had to drop crossmember to get more room to work. Short tail TH400. Removed tailshaft cover. There is a metal clip in the I.D. of the plastic Drive gear that you have to pull out with long needlenose pliers. Save metal clip. Mark shaft with a magic marker so you know how far to slide new Drive gear towards front of car during install. Slide old out, new in and install clip. I used a long punch to push clip in to lock Drive gear. Not a bad job ,worth the hour or so to get your speedometer reading accurately. JIM/Rott
Thanks for all the guidance everyone. I ended up building an excel sheet to crunch the numbers a little easier. Ordered the parts tonight - 15 tooth drive & 41 tooth driven with housing. I'll give everything a shot and see how it turns out.
