differential 8 is connected to the rear wheels 3 and 4 by means of half shafts 9 and 10 respectively the wheels 1 to 4 are provided with wheel speed transducers 11 to 14 respectively the transducers may for instance be of the variable reluctance type in which a fixed variable reluctance transducer cooperates with a toothed wheel rotating at the same speed as the road wheel so that the frequency of output signals of the sensor is proportional to the road wheel speed the transducers 11 to 14 may form part of a vehicle anti lock brake system as shown in fig2 the wheel speed transducers 11 and 12 for the undriven wheels 1 and 2 supply signals ul and ur which represent the speeds of the respective wheels to a circuit 20 for forming the average s of the undriven wheel speeds the signal s thus represents the vehicle speed the transducers 13 and 14 supply signals dl and dr representing the speeds of the driven wheels to adding inputs of subtractors 21 and 22 respectively the subtracting inputs of the subtractors 21 and 22 are connected to the output of the circuit 20 the subtractors 21 and 22 thus provide signals wsl and wsr representing the amounts of wheel spin of the left and right driven wheels 3 and 4 respectively thus the elements recited above relating to the generation of signals indicative of wheel spin of each of the driven wheels comprise a detecting.