I just bought a Specialized Sirrus and tried to hook up my Garmin 305 and found that I had to make a choice between cadence and speed. There was a really large difference between the pedal and the lowest bar that connects to the rear wheel. I tried using quarters and double sided tape to decrease the distance between the magnet and sensor, but that didnt help much.
Should I be looking for a different unit? I would like cadence, speed and HR, since I am biking to give me a better workout without the impact on my ankles and knees that running was giving me.