Fuel system

A fuel system for an internal combustion engine comprising a fuel pump, a controller for controlling the operation of the fuel pump so that fuel is delivered to the engine at a required time and in a desired quantity, means for supplying a demand signal to the controller, a wheel driven by the engine, a plurality of indicia on the wheel and positioned about an axis of rotation of the wheel, transducer means responsive to the passage of said indicia as the wheel rotates, means responsive to pulse signals generated by said transducer means for measuring time intervals between the passage of the indicia past the transducer means, wherein said measuring means comprises means for recording two time pulse sequences of signals from the transducer means at different engine speed and load operating conditions, means for determining actual indicia spacing errors from said two time pulse sequences of signals and means for calculating the engine speed using the actual indicia spacing errors and the measured time intervals between the pulses produced by said transducer means.