If your bike's speedo stops registering speed and your odometer does not move as far as registering miles traveled, you also realize that your directional signals do not self cancel, your problem is more than likely the VSS (vehicle speed sensor) on the transmission. It collects metal particles that are from the wear in the transmission being that it is a magnetic pickup that reads off the 4 th gear cog in the transmission. You can clean it off then replace it, but most times it will go bad again real soon. The best thing to do is replace it. I believe the cost is around $50. A simple repair requiring one bolt for removal. It's located on the transmission just behind the rear pipe. A good quality gear lube will all but eliminate this problem.
Also see this other post...HERE
And this post... HERE
The 06 bikes no longer advise use of Syn 3 in the trans from HD
( See Pic Below.)