I recently acquired a 1988 raider and the speedometer doesn't work. The guy I bought it from said that a few weeks before, the speedometer was jumpy and then stopped working entirely. My neighbor knows a lot about older cars and said that it sounded like the gear that drives the speedometer cable got chewed up. My Haynes manual calls it the speedometer driven gear and instructs that when removing the unit, to make note of what orientation it was in to begin with. I verified that the cable does not spin when I drive around.
1. Is there anything else that might produce the same results?
2.Why do I need to mark what orientation it was in? I of course read that warning after trying to unscrew the thing, realizing that it pulls straight out, and finding that I needed to drain the lube first anyways.
3. When I do drain the lube, how much should I expect to have to put in?

Thanks in advance