An energy saving device includes a control box, a battery, an operation setting system, a car speed sensor assembly, and an air intake fan motor. Thus, the IC chip of the car speed sensor assembly identifies the car speed information detected by the five car speed sensors of the car speed sensor assembly, and transmits the car speed information to the IC circuit board of the control box, and the IC circuit board controls and regulates the rotation speed of the air intake fan motor according to the grades of the car speed preset by the operation setting system, such that the rotation speed of the air intake fan motor increases the air flow rate by a grade when the car speed is increased by a predetermined constant value.