Cruise control will not work .

1968 dart cruise control will not work. what should I look for or check to find out why it will not working.
I was told that if my speedometer needle is jumping around (which it is ) the sensors in the unit pick up on that and will not engage cruise . Does that sound like the problem ? How do I fix that jumping speedo needle ? lubricate the casing on the cable or lube the speedo gear itself in dash with graphite?
Thanks for you help.