Sensor assembly and method for non-intrusively sensing instantaneous speed of an engine are provided. The engine includes a crankshaft have a respective end connected to an externally accessible bolt. The sensor assembly comprises a housing including a recess configured to affixingly engage the externally accessible bolt. The sensor assembly further comprises at least one motion-sensing device configured to provide a respective output signal indicative of angular motion information of the crankshaft. A signal transmitter coupled to the motion-sensing device configured to wirelessly transmit the output signal from the motion-sensing device.