Here in the UK I had a friend get caught speeding. The police could not believe that a bike could do 35mph in a 30 on the flat so were more interested in the bike than the offence. I have been warned by the police that even though I do not have a speedometer- doing 50mph downhill in a 30 is not strictly legal.

BUT- A policeman can stop and issue you with a ticket over here for speeding on a bike. He will need proof but with the radar traps- cars with cameras tied to the speedo and radar fitted- he can get that.