The invention discloses a fuel economizing device for a vehicle engine, which belongs to the field of fuel economizing devices. The fuel economizing device for the vehicle engine comprises a decompression lever and a control circuit, wherein a lever body of the decompression lever is provided with two grooves, an axial elastic collar is arranged in each groove, the lever body is provided with a screw hole, a valve rocker limiting adjustment stud is arranged in the screw hole, and one end of the lever body is provided with a torsional spring; a collector of a dynatron BG2 of the control circuit is connected with a key K3; when the device is used for a gasoline engine, a valve rocker shaft seat is a composite shaft seat which integrates a rocker shaft hole and a shaft hole of the decompression lever into a whole, and an air compensation channel is arranged between a carburetor and an air cleaner; a collector of a dynatron BG3 of the control circuit is connected with a vehicle power supply, a base of the dynatron BG3 is connected with a throttle valve K2, and the collector of the dynatron BG3 is connected with a base of a dynatron BG4. The device ensures that a moving vehicle begins the sliding on the premise of not arranging a neutral shift, and the whole fuel economizing procedure is finished without extra operation actions by drivers in an unconscious state.