A wheel speed sensor assembly includes a rotor formed with first and second protrusions arranged circumferentially on the outer periphery thereof in two separate rows at an equal angular pitch. The first protrusions are angularly displaced from the respective second protrusions by half the angular pitch of the first or second protrusions. The sensor assembly further includes a senor having two magnetoelectric devices each opposing the first protrusions or second protrusions. With this arrangement, the differential waveform produced by the sensor has a flat stepped portion between any adjacent rising and falling portions of the differential waveform. The flat portions make the rising and falling portions steeper and shorter. Thus, when the sensor vibrates in the rotational direction of the rotor, the differential waveform is less likely to overshoot the upper threshold or undershoot the lower threshold, which in turn prevents abnormal block pulses from being produced. The detection accuracy of the sensor thus improves.
