There are potentially two gears that can go bad. First is the gear on the inside of the speedometer housing. 10 tooth with a helical angle. (DOES NOT COME WITH THE METAL SHAFT) 6mm long, 7mm diameter

Second is the lead gear which is next to the #1 digit of the odometer. Check to make sure it is tight on the shaft. Using a small screwdriver, see if it spins on the shaft. If it does, you should replace it also.