Hi
,
maybe a smarter solution then using a reed switsch(problem contact bouncing at higher speed) is using a Single-Chip-Encoder, like the iC-MH/MH8, and measure the change between two absolute position readings from the magnetic rotary encoder, or the time between two zero pulse. You might look at this two application: http://www.ichaus.biz/wp2_simple_measurement and http://www.ichaus.biz/wp_boost_performance .
Enjoy your project work!