A fuel system for an internal combustion engine includes a controller (14) which controls the operation of an engine fuel pump (13). Signals are supplied to the controller by a transducer (16) which is responsive to the passage of teeth formed on the periphery of a wheel (12) which is mounted on the engine crankshaft. In order to generate an engine speed signal it is necessary to know the tooth spacing errors and these are determined from the transducer signals by using a cross-correlation technique to correct the transducer signals for interval variation due to cyclic fluctuation of engine speed following which the tooth spacing errors are calculated to be used in subsequent calculations of the engine speed.