The drive gear requires pulling the tail housing, but the driven gear is a matter of one bolt. You can change the driven gear without touching the drive gear, but if it's really far off, you may need to change both.
Pull the driven gear and count the teeth. If your speedo reads fast, put in a gear with more teeth, if it reads slow, you need less teeth.
It's really simple, but my speedo has been way off for over 2 years, and I haven't fixed it yet
