yeah i tried a mechanical speedo as well. it only lasted about 2 days. (this was a new "retro" style speedo which is 98% plastic.) and at 60 k/mh, the plastic gears in the drive mechanism broke. well, it read 60 k/mh...not sure how accurate it was, but i was hauling butt when the speedo drive busted.
the gears in it were literally disintegrated, and melted. i don't think it was ever meant to go as fast as i made it go. i'm petty sure that it was designed for a peddle only bike that might hit 30 k/mh tops.
if i could find a cheap old mechanical speedo with metal gears, and that is built like they used to make 'em, i think it would last.