A fuel economizer for an internal combustion engine includes a computer assembly, a battery, an operation setting system, a lead sensing fan, and an air intake fan motor. The lead sensing fan is mounted on the front of the traffic vehicle. The air intake fan motor is connected to an air inlet port of the internal combustion engine. The lead sensing fan is rotated during movement of the traffic vehicle. When the rotation speed of the lead sensing fan reaches a preset value, the lead sensing fan transmits a signal to the computer assembly which regulates the rotation speed of the air intake fan motor according to a speed grade preset by the operation setting system so as to control the air flow rate of the air inlet port of the internal combustion engine.