Disclosed is a method for controlling a speed of a vehicle combustion engine, the engine including at least one combustion chamber, into which a mixture of air and fuel is injected, and an air box, configured to inject the air into the combustion chamber and having an air flow rate controlled by a regulating butterfly valve, the regulating butterfly valve having a variable angular position, controlled by a predetermined position of an actuator. The method includes the steps of evaluating a so-called “load” resistant torque resulting from a plurality of external loads applied to the engine, determining, from the calculated load resistant torque, a position of the actuator, so as to determine an angular position of the regulating butterfly valve, and controlling the position of the actuator, so as to control the engine speed.