A method of controlling the speed of a vehicle provided with an internal combustion engine connected to driving wheels through a transmission; the control method comprises the steps of: determining, during a design stage, a table of motion that provides, as a function of cruise speed (V C), whether it is more convenient to proceed at a constant speed or to proceed at a variable speed cyclically alternating a period of active motion and a period of passive motion, and if motion at variable speed is more convenient the table provides a minimum speed (V MIN), a maximum speed (V MAX) and an optimal acceleration (A BEST) associated to the cruise speed (V C); storing, during the design stage, the table of motion in a control unit; determining a desired cruise speed (V C) of the vehicle; automatically determining, by means of the control unit and by using the table of motion, whether at the desired cruise speed (V C) it is more convenient to proceed at a constant speed or to proceed at a variable speed; piloting, by means of the control unit and if according to the table of motion it is more convenient to proceed at a constant speed, the internal combustion engine and the transmission of the vehicle in order to advance the vehicle at a constant speed equal to the desired cruise speed (V C); or piloting, by means of the control unit and if according to the table of motion it is more convenient to proceed at a variable speed, the internal combustion engine and the transmission of the vehicle in order to cyclically alternate a period of active motion, in which the internal combustion engine is turned on and transmits to the driving wheels a positive torque to increase the speed of the vehicle from the minimum speed (V MIN) to the maximum speed (V MAX) at the optimal acceleration (A BEST), and a period of passive motion, in which the internal combustion engine is stopped and disconnected from the driving wheels and therefore the vehicle moves forward by inertia at a decreasing speed from the maximum speed (V MAX) to the minimum speed (V MIN). determining, during a design stage, a table of motion that provides, as a function of cruise speed (V C), whether it is more convenient to proceed at a constant speed or to proceed at a variable speed cyclically alternating a period of active motion and a period of passive motion, and if motion at variable speed is more convenient the table provides a minimum speed (V MIN), a maximum speed (V MAX) and an optimal acceleration (A BEST) associated to the cruise speed (V C); storing, during the design stage, the table of motion in a control unit; determining a desired cruise speed (V C) of the vehicle; automatically determining, by means of the control unit and by using the table of motion, whether at the desired cruise speed (V C) it is more convenient to proceed at a constant speed or to proceed at a variable speed; piloting, by means of the control unit and if according to the table of motion it is more convenient to proceed at a constant speed, the internal combustion engine and the transmission of the vehicle in order to advance the vehicle at a constant speed equal to the desired cruise speed (V C); or piloting, by means of the control unit and if according to the table of motion it is more convenient to proceed at a variable speed, the internal combustion engine and the transmission of the vehicle in order to cyclically alternate a period of active motion, in which the internal combustion engine is turned on and transmits to the driving wheels a positive torque to increase the speed of the vehicle from the minimum speed (V MIN) to the maximum speed (V MAX) at the optimal acceleration (A BEST), and a period of passive motion, in which the internal combustion engine is stopped and disconnected from the driving wheels and therefore the vehicle moves forward by inertia at a decreasing speed from the maximum speed (V MAX) to the minimum speed (V MIN).