Best thing to do is fitting a bicycle computer. This can be mounted on the handlebar which also gives you a better sight on your speed without looking down.
I have fitted the Sigma Sport Targa M. The pickup was glued with 2 part epoxy to the brake caliper mounting bracked and as for the signal I substituted the supplied magnet for a very strong magnetic washer stuck and glued to the brakedisk.
For pictures:
http://img67.exs.cx/img67/1451/TachSav2.jpg
Greetz
Kropatchek