An adjustable radiator fan is designed to modify its speed relative to the engine's cooling requirements. An adjustable fan may be run through a thermostat, or some units may be controlled by an electronic control unit (ECU) that determines engine coolant temperature, engine load, and other variables. The ECU receives such information and either alters the voltage delivered to the fan motor, or the fan motor's speed is controlled by a feedback drive system. Some models might also offer the option of manual adjustment which allows the speed to be set by the driver or mechanic as deemed fit. Adjustable rated fans with advanced cooling systems can improve fuel economy because the fan operates less, better engine temperature control is achieved, and engine noise is reduced compared to fixed speed fans.