Arduino bike speedometer.
by amandaghassaei at Instructables
http://www.instructables.com/id/Arduino-Bike-Speedometer/
Radio shack magnetic reed switch.
http://www.radioshack.com/product/index.jsp?productId=12706327
speed vest project…
By Mykle Hansen at Make
http://makeprojects.com/Project/Speed-Vest/1344/1#.UHcZaa5U3Tq
Wheel sensor , from a wired (not wireless) bicycle speedometer (aka cyclocomputer) The sensor must perform like a reed switch, which is binary, rather than use the magnet on the spoke to vary the inductance. We used a Sigma Sport BC 500, which costs $15, but the city of Portland gives them to residents free, to encourage bicycling! Ask a bike shop if they can sell you just the wheel sensor part, without the computer and display.
Calibrating bike wheel sensors…
By Sheldon Brown
http://sheldonbrown.com/cyclecomputer-calibration.html
Arduino code for measuring reed switch and magnet
By Greg Borenstein
https://github.com/atduskgreg/bike_mountain_speedometer