Bouncing Speedometer...

I had this problem even after replacing the original speedometer cable. Turned out the new cable was just a little too long. When I clipped the plastic drive gear on the end of the cable sleeve it pushed the excess cable length into the sleeve and caused it to bind up and put extra resistance on the gear.

This extra resistance made the plastic gear hard to turn so the metal gear it mates up to in the transmission slowly chewed up the plastic teeth. This caused the needle to bounce and eventually the plastic gear teeth wore out and the needle stopped moving.

Bought a new cable, it was also too long. So I cut about 1/8" off the end that goes into the plastic speedometer gear. The other side was already bolted up to the back of the instrument panel. This time when I clipped on the plastic gear I could spin it freely with the tips of my fingers. Before it took some effort to get it to spin.

Now the needle is steady. Maybe a tiny bounce below 20 mph but not that noticeable. Around 30 mph or higher it is very smooth.

Also, put some ATF fluid on the plastic gear before putting it in the transmission.

So basically, make sure the plastic gear spins freely with very little resistance.

 

I've never thought to check for that. One of my Fords still bounces a little,only between 15-20mph,even after I greased the cable. The core slides easily in and out of the cable.