Abstract: The speedometer of a stationary vehicle is evaluated for accuracy and calibrated regardless of vehicle tire size. The powered driven wheel or wheels is/are disengaged from road contact and a detachable housing member with a permanent magnet therein is releasably threaded onto the tire valve stem. A sensor associated with a testing unit is positioned adjacent the housing member so that as the wheels are driven at selected speeds, as indicated by the vehicle speedometer, a train of electrical pulses derived from the sensor is delivered to the testing unit at a rate proportional to the wheel rotative speed. This pulse information, in conjunction with a compensated reference signal generated by a scaler circuit, is processed by computing circuitry in the testing unit resulting in a digital display indicating the actual vehicle speed. The scaler circuit includes a tire size selector providing the testing unit with a number of reference frequencies corresponding to a range of selectable tire sizes.