A speed sensor (1) comprises a first magnetic sensor (3) with a first magnetic detector (30), which is connected to an electronic circuit for conversion of the output analog signal to a digital signal. Upper and lower threshold values of the output voltages from a first magnetic detector (30), upon excess of which a first analog signal is converted to a first digital signal, are self-adjusting with respect to upper and lower peaks of the analog signal, and the south pole (S) of a first basic magnet (31) engages the first magnetic detector (30). A second magnetic sensor (4) with a second magnetic detector (40) is connected to an electronic circuit for conversion of its output second analog signal to a second digital signal. Upper and lower threshold values of the output voltages from the second magnetic detector (40), upon excess of which the second analog signal is converted to the second digital signal, are self-adjusting with respect to upper and lower peaks of the analog signal, and the north pole (N) of a second basic magnet (41) arranged parallel but with opposite orientation to the first basic magnet engages the second magnetic detector (40). The distance (D) between axes (03 and 04) of both the magnetic detectors (30 and 40) is equal to or less than the width (W) of a tooth (201) of a sensed gearwheel (20) or the width of a magnetic sign of a sensed disc. The sensors are arranged such that a disturbing external magnetic field will only bring one of the magnetic detectors outside its range of sensitivity so that it will stop to output a signal. The other sensor will continue to operate, and its output signal will still correctly represent the speed of the gearwheel.