Or just find the local taxi and police measured mile. Ride it a few times noting the pre or post wheel rotations to catch up/slow down so that both the odometer and the mile stop points are recorded and averaged. Then do the math for the difference and correct the computer calibration.
I did this a few times one year and found the same bike (and tires) had different results... There's something to the simplicity of believing in side walls and a chart
Andy.