A sensor wheel (10) is used, for example, to detect variations in the speed of an internal combustion engine with a high degree of accuracy so that an erratic running value can be calculated from these variations. Such a sensor wheel comprises three segments, for example, which ideally are of equal length but in reality are usually of slightly different lengths. In order to avoid measuring errors owing to these differing lengths when detecting variations in the speed, the actual lengths can be determined by a known adaptation process. The device of the invention is constructed such that it has a plurality of adaptation filters (17.B1 to 17.B4) for each segment in order to detect different adaptation values for different speed ranges since it has emerged that different lengths are measured in different speed ranges, which is an error which has to be compensated for, since the length of a segment is actually permanently constant.