Speedometer gear

Hi all. I just purchase my new baby - 1978 w/ 3spd auto. The speedo isn't working so I disconnected the cable at the tranny and did the drill test. Speedo worked perfect that way, so I removed the transmission driven gear and sleeve, then spun it that way. Again works perfectly. The gear itself looks to be in perfect condition- no mangles or broken teeth. I had no way to look inside the hole, so I just felt around with with my finger. I couldn't actually feel anything that felt like threads or teeth that the gear would be driven by. Of course, when I put it back together it still didn't work. I'm thinking the speedo gear internal to the tranny may have broken off or ??? Should I be able to feel or see it through the hole? Do these things break? (I still can't believe these kinds of parts are made of plastic.). Is there some other way to inspect that piece? Also, if I do have to replace it, does anyone know how much ch of a project it will be? I've never actually opened up a transmission before. Thanks!

Well, this is a Joel(adam's apple) question,but i'll try to fill in till he can give a proper response(he rebuilds these things).. the speedometer gear that you are trying to feel for is on the output shaft, i have a picture, and the gear you are feeling for is the gear that is colored teal-ish.  I think(dont hold me to it) that this gear is metal,  and the driven gear as you found out is plastic. Probably in the event something happens with the cable it would strip that gear(which is easier to replace) than the gear on the output shaft...  

IIRC, the gear on the output shaft is pressed on.

basically  it is pressed/slid onto the output shaft.... and held in place with a retaining clip

Thanks for the video reference StingrayJimJr!  Doesn't look too difficult while the tranny is sitting on the bench.  I'm sure it will be much more exciting if I need to do this without dropping the tranny completely. :)  YouTube "suggested" a couple other videos, and I found this one which helps you figure out which Drive Gear and Driven Gear combo is needed to get accurate speedo readings. 

Good luck !!  not so sure you can remove the tail shaft while the transmission is in the car, but, i suppose it can be done just not a lot of room to work on anything under there... might have to unbolt the transmission crossmember and let the engine and transmission tilt downward.... just be sure to brace everything, lets not make more work out of it :)

My '72 was easy to do, only had to jack up enough to get under the car. I was only putting in a new o-ring and seal; would've ordered a new plastic gear too if I had known it was a good idea to replace it as well ahead of time. I left the old seal on and added the new one on top of it.