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
