Assuming you have a computer (and how else would you have posted here?), you can do it for about $5. All you need is a cheap USB mouse. I've tried it; it does work.
-rb
It seems like this post moved away from the reed switch discussion, but maybe someone will get some use out of the idea below.
I bought a $10 bike computer from Wal Mart. I used a neo magnet instead of the one that came with it.
Set the tire size on the comp to 1000. Every time the mag passes the comp sensor it adds one to the count.
I think the problem mentioned with the reed switches is that once it gets going really fast the switch just can't reset before the magnet makes it around again. My other problem with them was they seemed to become magnetic rather quick and would just stay closed or the arms bent and made them stay closed.
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