Abstract: The present disclosure relates to a controller (7) for controlling an electric machine (6) to drive a wheel (4) of a vehicle (1). The controller (7) includes a processor (15) configured to determine an effective torque (T). A speed demand signal (27) for controlling the wheel speed is output by the processor (15). The processor is configured to detect changes in the effective torque (T) as the wheel speed (S) changes and to modify the speed demand signal (27) in dependence on the detected changes in the effective torque (T). The processor (15) may determine a derivative (dT/dS) of the effective torque (T) with respect to the wheel speed (S). The present disclosure also relates to a method of controlling an electric machine (6) to drive a wheel (4) of a vehicle (1).