Check the condition of the square hole in the drive gear in the transmission tailshaft. If the hole gets rounded-out, it won't turn the cable the right speed. It's also possible for the square shank on either end of the cable to wear to the point that it slips. I always replace both the inner cable and the housing as a unit, as internal roughness in an old housing can cause erratic cable speed, speedo needle shudder, etc.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!