This is a fairly commom problem with some Yamahas. The most likely culprit is the speedometer "key" that's located in the speedometer cable housing that's attached to the drive axle beneth the secondary clutch. Pull the three or four nuts that attach the housing to the chassis. In the inside center of the housing is a small braided wire square key that's about two inches long that goes into a brass bushing pressed into the end of the drive axle. These wire keys can start to come apart after awhile, and once they do your speedometer doesn't have a signal. I've seen where this key can either be left out when putting things together after a track change, or kinked when putting things back together, which will also cause it to come apart. I've also seen the brass bushing get rounded out, so that the key just spins inside it. That's probably why Yamaha went do a magnetic pickup for the speedometer in '02.
As a disclaimer, none of the above things have ever happend to me.
