I have an 82 Z28, 5.0. PO replaced the 200 with a TH350 out of a late 70s Monte Carlo (he thought). Speedo is way off, 15mph faster than actual speed (going 60 actual, speedo shows 75). Trying to figure out what drive and driven gears I need. I have a set of 18-22 driven gears (blue 20 in there now) for a 1.84 drive gear. Doing the calculations online and knowing my tire diameter, and axle ratio is 2:73, I couldn't figure out why I was so far off. Took out the driven gear and there's a medium bright blue (no mistaking it) drive gear on the output shaft. None of the Chevy gears seem to be blue (7-10 gears) but options are pink, orange, green, and purple. Pretty sure it's not purple because if it were I'd need a 21 driven gear to make the speedo accurate and it's off way more than that. The only blue I see is 18 tooth BOP. My tailshaft housing has the small driven gear bullet hole in it, not the larger 2 inch BOP one.

So confused. I'm thinking I'll just get a 9 or 10 drive gear so I can use my existing set of driven gears, but not sure if the drive gears will fit on the output shaft. I have no idea where this trans came from or if it's Chevy or otherwise. Are the output shafts the same size and will a 9 or 10 tooth fit on a shaft that has an 18 on it now? I'm guessing they are all 1.84 OD gears? Thanks

 

The inner diameter of the gears is the same between the two transmissions.

 

Thanks, I just ordered a a green 9 tooth. But then I saw this one on TCI's page:

That's definitely the color, if I can go by color. I might just take it off and count the teeth to be sure. The speedo is way off, and if it's a 10 tooth than that would not explain it as the calculator says that a 10 tooth would require a 21 driven gear (I have a 20 in there now and it would not be off by that much). Maybe I also need to take the diff cover off and make sure no one has changed the drive ratio from the standard 2:73. Is the blue 18 tooth a 1.84 OD drive gear too?

 

Figured it out. The drive gear is a 10 tooth - I took it out and it has it stamped right onto the gear. Purple/blue - I must be color blind. It's considered purple but it looks blue to me.

Also took my diff cover out and the ring gear had 42:13 stamped on to it, so it's a 3.23. Lucky me, the car has the upgraded axle on it.

So when I plug all of that into the TCI calculator (10 tooth drive gear, 3.23 axle, 26" tire diameter), I get a 25 tooth driven gear requirement to make the speedo accurate (25% off), which is perfect because I'm running a 20 and each tooth added is about 5% on the speedo. Of course I can't get a 25 driven gear for a 1.84" OD drive gear so the 9 tooth drive gear I just ordered brings me down to a 22 driven gear which I have in my set.

I've learned alot about speedo gears through this. Thanks for your replies.