The cable may be slipping.

Remove the cable from the speedo/tranny and see if the ends are still square. Make sure the cable connections are tight on the back of the speedo and the transmission tailhousing. Be sure the bends of the cable are not excessive, it should be straight as possible.

The speedo portion of the gauge works by magnetic drag on a cylinder near a magnetic drum that the speedo cable turns. Only the odometer is run by gears.