how do you measure the exact wheel circumference (for use in combination with a speed sensor)?
I mean, it gotta be a bit less, when you ride and put weight on the wheels, no? 
I just used something like this.
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Dab drop of paint on tyre. Roll slowly along path for couple revolutions of wheel with you sat on bike. Measure distance between paint marks on path and take average. Then clean tyre quickly before paint dries too much!
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I just roll the tire onto the tab of a tape measure & measure the diameter of the wheel. Multiply that by pi. Bingo, bango, done. +/- 5mm. Round up if you want to make yourself feel good.
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thanks everyone 