To determine the rotary speed of a toothed element (1), for example the starter gear of an internal combustion (IC) engine, or a toothed wheel coupled to a wheel of a vehicle for evaluation of the speed, and change in speed thereof, for use in a brake anti-block system, a magnetic field is generated, for example by a permanent magnet (3) which has both radial and tangential components with respect to the teeth (2) of the rotary element. A magnetic field dependent sensor (4) such as a Hall generator or a thin-film permalloy-type magneto-resistive sensor is positioned to respond to the tangential component of the magnetic field, to thereby eliminate shifts in the null or center line about which the field strength undulates as the disk rotates to permit ready evaluation of the output from the sensor by threshold circuitry.