disclosed is a rotational speed sensor comprising a rotatable ring . the rotational speed sensor has k magnetic pole pairs distributed angularly over the rotatable ring . also , sensors are positioned relative to the rotatable ring such that a varying magnetic field is detected . the sensors comprise a first pair being positioned 2πl / k radians apart . the sensors further comprise a second pair being positioned 2πm / k radians apart . the first pair of sensors and second pair of sensors are positioned at a relative position of */ 2 radians . l , m are integers between one and k and n is an integer greater than one .