1967 Chevy Chevelle spedometer

If you just installed a cable an ddidn't check the housing for kinks, or such and didn't lube the cable this could be the cause. Make sure it is tight at both connections as well. Theer could also be abiniding in the cable itself so y ou may have to relace the whole cable assy. Also your speedo may be defective, you can take that to a speedo repair place and they can tell you if there is something wrong with that.