I put the polar cs200cad on my bike. It's great, it comes with rubber bands as well as zipties. So when you are sick of the false readings it gives out, it's rea quick to get it off and put back on again later on to "give it another try"

Seriously, just look at the chart that would have come with it, punch in the wheel size and go. I was sitting next to a friend yesterday in the park and we both were showing 31.3 kmph. Close enough for me. I thought it dodgey after the 1st day I put it on and went and did some hills it came back with max of 92kmph.