Dodge Dart 74 speedometer cable troubles

[creamer]

I have installed a new speedometer cable only to find out that it is doing the same as the old,,, the speedometer needle in the gauge is jumping , what could be wrong?

 

[Mexican Spec]

I have the same problem with my 1974 Dart. In the lower speeds like 40 to 60 it jumps a lot, but at the higher speeds like 120 to 145 it looks solid.

I need to know how to fix it too. I need to know if it is the speedo head or the plastic gear inside the transmission.