A device and a method for controlling vehicle speed in a vehicle equipped with brake cruise control when the vehicle is travelling downhill are provided. The method involves driving a vehicle downhill with the brake set speed set to a first brake set speed; detecting a current vehicle speed; automatically applying a brake torque using at least an auxiliary brake to maintain the first brake set speed; and detecting a manual application of a vehicle service brake, in order to decrease vehicle speed. If a control unit detects that the driver is applying the service brake, then the control unit is automatically arranged to set the brake set speed to a second brake set speed that is lower than the first brake set speed; and to apply a brake torque using at least the auxiliary brake if a detected current vehicle speed exceeds the second brake set speed.