The other thing that throws off the calculation is that speed and distance are not measured directly (unless you use GPS). Instead, both speed and distance are calculated backwards from wheel revolutions (on an ABS bike, which has sensors on the wheels), or even countershaft revolutions (non-ABS bikes).
This means that tire circumference changes (e.g. replacing old, worn tires with new ones) also lead to apparent mileage changes. Ditto for changes to front/rear sprocket tooth count on non-ABS bikes.
On a 19" tire, 5mm thread wear leads to a reduction of circumference by a fraction over 1%.
Last edited by BackPacker; 08-14-2017 at 08:10 AM.