The method includes, during a first step (8) determining in real time the fuel consumption gain of the thermal engine by defining the difference between the consumption of the thermal engine in operation and the consumption of the thermal engine when stopped independently from other energy sources outside the thermal engine, the equivalent consumption being calculated from a predetermined coefficient corresponding to the ratio between a consumption difference and an engine power difference for a given engine speed; during a second step (9), deciding to switch on or switch off the thermal engine according to a criterion and to the consumption gain, the criterion depending on the recovery level of the electric energy recovering means and being determined in order to increase the use of the electric energy storage means and to switch off the thermal engine as often as possible.