Which speedo drive gear do I need?
 

The speedometer in my 81 Z28 isn't working. I've disconnected the cable from the cruise control module and spun the cable with a drill and I've done it from the transmission up too and the speedo spins both times. Now when I originally took off the old speedo cable, there was some kind of blue plastic on the end that was all twisted up and I assumed that was from the old speedo gear getting torn up. So since the real seal on the trans tail shaft was leaking I figured it was a good idea to take the tail shaft off and fix the leak and replace the drive gear that I thought was all torn up. Well I took the tail shaft off and to my surprise, the drive gear and the driven gear are both in perfect shape. Now I slid the speedo gear on the cable and it does grab, so the cable isn't spinning inside the gear. Any thoughts where to check next. Also, I put in a new ring and pinion and went to 3.73 gears so I need to change the drive gear anyway. Anyone know what speedo drive gear or drive/driven gear I need?

If the "blue plastic" was all twisted up, then the gear might be good, but that blue plastic is part of the gear, so it needs replacement. If you changed to 3.73 gears, then you need to change gears. Here's a chart that will help you choose a gear. It's not as simple as just picking a gear for a certain rearend ratio!
Speedometer Gear Calculator and Charts - TCI® Auto

I'm found out quickly that it's not just a simple gear change. I've been told that I need to replace my drive gear and driven gear. Here's the problem though. My driven gear housing is the large type that looks like the one from a 700r4. Now that housing holds a 35 to 39 tooth gear and I've been told I need to go to a 22 tooth gear. Now will that 22 tooth gear fit in the large housing? The smaller housing says it's for that gear size. So if I need that smaller housing, where do I go for a short tail shaft for a TH350 with the smaller opening? I can't be the only person who has run into this.

i did a 3.73 gear swap on my last 81 i asked the mechanic he said he did not chage the drive gear just the idler gear but i eventually had the same problem of speedo not working but the cable mated properly to the idler when i put it back together it worked??? i thought it was wierd but oh well its works then two months later it quit so i went under again and found it was just sorta wiggling its way way out so i used "form a gasket" to sorta glue it back together and 4 months later when i sold it it was still working. thats kinda redneck but hey it worked