A system and method for controlling cooling an engine involve an engine coolant circuit with a radiator and an engine, a fan and a coolant pump are provided. The fan and pump may be electrically driven, driven by a variable speed clutch, hydraulically driven, or driven by some other actively controllable means. When an increase in heat transfer rate is indicated, the fan speed or the coolant pump speed may be increased. The choice of increasing the fan speed or increasing the pump speed is determined so that the power consumed is minimized. dQ/dP, the gradient in heat transfer rate to power, is determined for both the fan and the pump at the present operating condition. The one with the higher gradient is the one that is commanded to increase speed.