Data signifying the value of a controlled parameter which varies as a function of engine speed is stored on a disk rotating with an engine shaft. A sensor senses the data starting with a predetermined reference position and extending over a predetermined fixed scanning time. Since the time is fixed, the angle of rotation covered by the sensor will vary as a function of engine speed. The data is so arranged along the disk that the value of the parameter at the given speed can be determined from the sensed data at the end of the scanning time. The data is stored either as markings to be sensed serially from the start to the end of the scanning time, or is stored in parallel tracks to be sensed simultaneously at the end of the scanning time. For straight line approximations of the characteristic curve specifying the variation of the parameter as a function of speed, the disk has tooth-like segments indicating the required slopes, intervals for which the slopes are constant, and sign of the slopes by the length of the segments and/or the interval between segments.