I understand there are a few factors that affect the reading. I followed the TrailTech instructions (reference the picture they enclosed) and mounted the sensor in the same spot as they indicated. The cable is too long, but I looped it and tucked it behind the front # plate out of the way (using a cable tie to hold it). Then did the following to set it up: Marked the tire and driveway with a chalk. Got on the bike and got the 'minister of home affairs' to mark the driveway after 5 (yup 5) rotations with me on the bike. Measured it with a tape measure (i think it was 423 inches). Multiply this by 25.4 and divide by 5 gave me 2149 - this is the # is used. Factors affecting the measurements above: Tire size 80/100-21 Dunlop D739 Inflated to 15 psi Tread approx 95% I have only checked my measurement against my car over a mile, and it was 'close enough for government work' as far as I could tell.