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Geometric circumference is pi times diameter, but the rolling circumference is about 3% less because the tyre deflects under load. That rolling figure is what determines revolutions per kilometre and therefore speedometer and odometer calibration.
Rolling circumference
diameter = rim x 25.4 + 2 x width x aspect/100; rolling circumference approx pi x diameter x 0.97
The contact patch flattens, so the effective radius is slightly smaller than the free radius. Manufacturers publish measured rolling circumference for this reason.
In speedometer calibration, ABS wheel-speed reference values and gearing calculations. It is stamped in tyre specification tables.