But why buy another speedo when the one you already have can be set to be accurate by putting a sat nav on your bike for ten minutes, seeing how much your speedo is out compared to the sat nav speed and when you stop putting a little bit of tape or a marker at the real speeds.
so if your doing 30 on the sat nav and your speedo shows 33 just put a 30 sticker at 33 the same with 40 and so on.
Its not rocket science, you cant get more accurate than satelite tracking.