Speedometer jumps around

The speedometer in my 47 Dodge jumps around and even shows an increase in speed as I am slowing down. It also pegs out on the high speed side while at highway speeds. The cable and speedometer are original. I just installed a new center disk and lightly cleaned and lubed the speedometer. Everything seems free. The cable is in good shape. I pulled out the inner cable and lubed up with the correct lube. I tried another speedometer and it did the same thing. The 727 tranny has a 30 tooth gear and looks in good shape. So the question is, does anyone know if I can use a 47 speedometer with my 727 tranny and 8.75 rear end?

Have you had any luck with your speedo? Mine on my 71 Cuda was working fine, then when driving it Jumped from 60 mph to almost 90 mph while cruising down the highway. It is now very sensitive and jumpy and seems to be off 10 to 20 mph faster. I haven't had a chance to look into it. Was hoping I could find some direction here.
Thanks

I ended up hanging a speedometer from a 72 binder under the dash until I can find a 5" round one to replace the original. If I had $250 for a new one, I wouldn't still be looking. Since the "hanger" is reading steady, I believe the origial one is worn out. At least I know the cable and tranny gear is working correctly. I had a similar issue with my 66 Newport when is was under 50 degrees. I ended up removing the cable from behind the speedometer, pulling out the inner cable, coating it with cable lube from NAPA, and reinstalling it. No problems since.

I am using the original speedo on the 36 with a 518 trans. The rear is a B body 8 3/4 with 3.55 gears.
31 tooth speedo pinion. It works well within about two miles per hour. I would test the cable by removing from the trans and turning with a drill. That would isolate which end of the cable the problem is. Also check the squareness of the cable ends and their matching drive holes. The cable could be skipping in the square hole.

Get a copy of Street Rodder magazine, in it will be ads for companies that will make you a new cable and companies that can rebuild your speedometer. Had one of them do my 64 300 20 years ago.

http://deluxespeedometer.com/

Deluxe Speedometer and Radio will fix your old speedo and build you a new cable for a reasonable price. They have done work for me and my friends and have a very good reputation. Well known in the Denver, Colorado area for at least 50 years.

R.