Mechanical gauges use gears and typically a rotating cable to determine the number of rotations of the wheel. The size of the mechanical gear and the number of teeth on the gear is mathematically calculated to determine the speed. Electronic gauges use sensors to accomplish the same task. These sensors usually involve a pick-up and a magnet that sends a signal to a computer on the motorcycle. The number of signals is translated to a speed reading or measurement that is displayed by your speedometer.