This is true on many bikes, but not the FZ6. My XR650L, and Ninja are attached to the front wheel, but the FZ is attached somewhere else. I'm assuming the transmission, but I'm not 100% sure on that. I can tell you 100% that it's not the front wheel though.
The FZ6 speedo is measured off of the transmission output shaft.
Any gearing changes will throw it off.
It's usually off about 9%~ even with stock gearing, so you should buy a Speedohealer if you plan on doing drastic gear changes.