I wouldn't rule out the cable , mine was jumping some and I was able to pull the cable off and lube it and it did cure my problem and they say you might even be able to order an original as there are some still out there I think this is the original number 4897646AA but you may want to check
And the cable is usually the cause from my own experience but you could have another issue , I found this info
from tachman here is a link it may be helpful
http://www.tachman.com/tshoot.htmthis is info in the link
NOW, LET'S TROUBLE SHOOT YOUR SPEEDOMETER. YOU WILL NEED A REVERSIBLE DRILL (0-2800 R.P.M DRILL IS THE BEST) BECAUSE SPEEDOMETERS TURN COUNTER CLOCKWISE. THERE CAN BE SEVERAL PROBLEMS WITH A CLASSIC CAR SPEEDOMETER. THE CABLE, THE SPEEDOMETER AND THE GEAR IN THE TRANSMISSION, WHETHER AN AUTOMATIC OR MANUAL. TAKE THE CABLE LOOSE FROM THE TRANSMISSION AND CHUCK IT UP TO THE REVERSIBLE DRILL AND HAVE A FRIEND SIT IN THE CAR. POWER UP THE DRILL AND IF THE SPEEDOMETER JUMPS AROUND AND IS UNSTABLE, THE PROBLEM COULD BE THE CABLE OR THE SPEEDOMETER. REPLACE THE CABLE AND IF PROBLEM STILL EXISTS, THE SPEEDOMETER COULD BE THE PROBLEM. TO VERIFY THIS YOU WILL NEED TO PULL THE DASH, REMOVE THE SPEEDOMETER ( YOU WILL NEED TO CUT OFF AN OLD SPEEDO CABLE INSERT END) CHUCK THE END IN THE DRILL AND SEE IF THE SPEEDOMETER IS ERRATIC. IF NOT, IT IS THE CABLE OR THE GEAR IN YOUR TRANSMISSION. IF THE SPEEDOMETER IS ERRATIC WITH THE DRILL MOTOR ATTACHED TO IT OUT OF THE CAR, THE PROBLEM IS THE SPEEDOMETER. YOU CAN THEN SEND THE DEFECTIVE SPEEDOMETER TO H&H AUTO. CALL THE TACHMAN AT 864 895 4404 OR EMAIL US AT
tachman@bellsouth.net WITH ANY REPAIR QUESTIONS!
NOTE: 1970-74 MOPAR E BODY CLASSIC CARS ARE NOTORIOUS ABOUT DEFECTIVE GEARS IN THE TRANSMISSION CAUSING APPARENT SPEEDOMETER PROBLEMS!
