has it ever worked before? it could be that the slotted part of the cable end isn't engaging the gear at either the gearbox on the front axle, or up in the instrument cluster. i had this problem before, but it was my fault for not making sure the cable completely engaged the gears when i reinstalled it.
Motorcycle: GSXR600, GSXR750, GSXR1000, GSXR1100 - yup, got a full hand!
If all else fails you can fit a cycle speedometer for about $30 (Little more than a replacement cable). About 3 hours work if you need to make brackets and the advantage is that you now have:
at least......even on the most basic models.
If you calibrate it properly, it will be far more accurate than the stock speedo.