For those of you not already aware...
Mounted at the end of your machine's driveshaft, opposite the chaincase, is the speedometer housing. On most machines this housing acts as a grease cap for the bearing also found at that end of your driveshaft.
There is a mortised hole in the end of your driveshaft that turns a square key linking it to the speedo.
If your speedometer stops working the probable cause is the bearing failed.
If you drive your machine, say, an additional 500-600 miles on a fried bearing you'll do additional damage to your machine.
On most machines you must remove your secondary clutch to check this but the secondary comes off as a complete assembly and it's not nearly as intimidating as it first appears, so there's no excuse for not checking this regularly.
Ask me how I know....