A device and a method for controlling vehicle speed in a vehicle equipped with cruise control while traveling downhill. The method involves driving a vehicle downhill with the cruise control set to a brake speed; detecting a current vehicle speed; applying the auxiliary brake to maintain the brake speed; detecting a request for a downshift from a current gear to a lower gear; and determining if a downshift is permissible at the current speed. If a downshift is not permissible, then a control unit automatically applies the service brakes to retard the vehicle speed; retarding the vehicle from a current speed to a lower speed at which a downshift to a lower gear is permissible; and performing a downshift to a lower gear when the second vehicle speed is reached. The cruise control is set to a second brake speed lower than the first brake speed.