Rolling Radius is the effective radius of a loaded, rolling tire, derived from the distance it actually travels in one revolution rather than from its unloaded height. It is always smaller than the free radius because the tire deflects at the contact patch, and it changes with inflation pressure, vertical load, speed and construction, so it must be measured under the conditions of interest. Rolling radius sets the relationship between wheel speed and road speed, and it therefore determines the effective final drive ratio, speedometer and data-logger calibration, and the torque the tire applies at the ground for a given axle torque. Differences in rolling radius between the two driven wheels also force the differential to work continuously, and across an axle they show up as a pull or as data channel errors.