Transmission Tachometer shafts are components of a motorbike's transmission that measure the engine's RPM and transmit it to the tachometer. The shafts are usually attached directly to the gearbox or engine crankcase and use magnetic fields or other sensors to detect the engine speed. Accurate measurement of engine speed is important to keep the engine in an optimal power range and minimise wear on components. Transmission tachometer shafts need to be checked regularly and replaced if necessary to ensure an accurate RPM reading.