To measure tire circumference, first check your tire pressure. Then stretch a tape measure out on a suitably long bit of floor, street, or driveway. Put the valve stem at the end of the tape, a couple inches away from it. Sit on the bike. Roll forward, pushing with you feet. Check the measurement when the valve stem comes around again. To make that finer, ride a long known distance, a RWGPS route for instance. Modify your number by the percentage difference between the known distance and your reading.
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