An efficiency improving device includes a phase compensating circuit, a filter circuit, a first diode in a forward direction, and a second diode in a reverse direction; the phase compensating circuit includes two wires, and several parallel capacitors connected to the wires; the wires are connected to the battery of a car; the filter circuit includes a resistor, and capacitors connected in series with the resistor, and it is connected the phase compensating circuit; the first and the second diodes, and the resistor of the filter circuit are connected in parallel; the filter circuit is used for filtering off impure signals produced by the circuits of the car, thus improving performance of electronic equipments and sensitivity of the acceleration pedal and the automatic gearshift; the first and the second diodes are used to control charging and discharging of the capacitors, thus preventing the capacitors from making instantaneous and massive discharge.