Abstract: A method of measuring the instantaneous shaft velocity of a rotary machine having a shaft with a plurality of circumferentially spaced markers thereon, comprises the steps of sensing the passage of the markers with at least two circumferentially spaced sensors mounted adjacent the shaft to generate pulses in response to the passage of the markers; for each of the sensors continually deriving from the pulses and estimated spacing signals representing an estimated spacing of the markers, a set of angular velocity signals representing the instantaneous shaft angular velocity at a given point in time t.sub.i ; and varying the estimated spacing signals on an iterative basis to attempt to match the sets of angular velocity signals derived from the respective sensors. The described method allows a more accurate ICAV (Instantaneous Crankshaft Angular Velocity) waveform to be produced than was possible using prior art techniques.