If you notice your speedometer is jumping around or not working at all chances are your speed sensor is wore out, or more accurately the nut that drives the sensor wont make contact with the sprocket bolt anymore.
Locate the Speed Sensor it is right over the front sprocket
Remove the 2 bolts with an 8mm nut driver
Speed sensor pulls right out
Replace the nylon nut driver the old one prys right off the shaft
Old worn out nut driver the nylon part would not make contact with the sprocket bolt.
It is a simple 5 minute fix and the replacement nut costs around 5 bucks