Speedometer accuracy is very dependent on tire diameter, which is affected significantly on a motorcycle by pressure, load, and temperature. These values are strictly defined in the regulation's testing procedure, and the speedometer calibration must take these potential alterations into account. In addition, the manufacturers must consider the owner changing to non-standard tires, which may have a different diameter than the OEM fitment. In our last tire test, diameter of the seven rear tires tested varied by 2 percent, which directly affects the speedometer by the same amount. This also accounts for some of the discrepancy. Even with all these variables, however, the manufacturers do seem to be erring on the side of caution so the speedometer doesn't read low.