My speedometer does not currently work on my 1975 L48 4 speed coupe with the whole assembly installed. It has a 3:36 rear end and 255/60/15 tires. I pulled out the driven gear and it is a RED 21 tooth. I got under the car and confirmed that the speedometer gauge will move when the cable is spun from the 90 degree adapter that screws on between the driven gear "bullet" and the speedo cable. I have also confirmed that the speedo driven gear appears to get "pushed out" and does not spin when I observed it with the cable and adapter removed. So the issue is at the drive/driven gear interface. Where it gets weird, is that the driven gear is nearly perfect. No teeth are missing and only the slightest bit of wear was found on the teeth. The square hole is not rounded out in the shaft. The drive gear on the output shaft seemed fine, but I didn't check to see if it is loose or get a close- up to really see its condition.

I have a video I will post up later today showing the driven gear skipping initially, and then not spinning at all.

Has anyone experienced this issue before? Is it possible that the drive gear in the trans is spinning freely on the output shaft?

 

Update, I forgot I had a video on my phone that shows the drive gear. From what I see the drive gear looks pretty cooked. Let me know what you guys think:

 

Looks worn to me, easy enough fix I believe pull tail shaft housing and replace

 

I don't think the tail shaft housing will come off without disassembling the trans.
The Muncie and Saginaw 4 speeds have a retaining ring, like a large circlip, holding the bearing and tail shaft into the tail shaft housing.

I don't know about the T10's, assuming that's the trans in the picture.