The wireless ones are subject to interference problems. I have gone 85 mph at a stoplight, and ridden 10 miles at 0 mph.
The wired ones are more reliable and mine seems to match really well with roadside speed displays.
Couple caveats - your wheel size has to be entered accurately, and they weren't really intended for speeds much above 50 mph. And I don't know how you would attach the magnet without spokes but I'm sure someone will be creative with some epoxy or something.
from what I've read the Sigma bicycle units WILL work reliably for high speeds, also that rare earth magnets from Radio Shack can be glued to the wheel in place of the Sigma magnet....have not set one up on an MC yet but they have worked great on my bicycles...
