There could only be 2 problems. It's either a broken cable or stripped gears. Where the cable goes into the transmission, there is a gear that spins the speedometer cable. It is made of plastic so it will wear down and eventually stop spinning. That's about all it can be.
FYI It can't be the wires because the cable spins the speedometer first, then that also spins the odometer. The computer picks up a signal from the speedometer, there is another sensor in the speedometer that picks up the signal and tells the computer when to shift.
Mike