A method for monitoring an extra-fuel consumption and for determining a cause of an extra-fuel consumption of a terrestrial vehicle, in particular a commercial or industrial vehicle, comprising at least the following steps in succession: - (Step 1) a first acquisition of at least. a first set of input parameters related to environmental and mission conditions,. a second set of input parameters related to vehicle specifications and. a third set of input parameters related to a vehicle driving style; said vehicle "driving style" meaning those conditions under which the vehicle is used by the driver, both negative and positive accelerations applied to the vehicle, the frequency of use of the brakes, the use of the cruise control, the optional use of the Stop & Start; - (Step 2) estimating a first expected fuel consumption in time, based on said first, second and third set of input parameters, - (Step 3) a second acquisition of a measured fuel consumption in time, - (Step 4) comparing, preferably in real time, said expected fuel consumption with said measured fuel consumption, thereby obtaining an error value in time, - (Step 5) checking: whether said error value increases abruptly (5 = Yes), if so - (Step 6) activating a flag indicating an extra-fuel consumption condition; said method further comprising performing the following steps in succession when said flag is active: - (Step 7) acquiring a fourth set of input parameters related to stored records relating to events concerning abnormalities and variations in the operating conditions of the engine and/or vehicle, - (Step 8) checking: whether at least one record is present, then - (Step 9) the instant of memorization of said record (of each of said records) is compared with said activation of said first flag so as to identify a cause-effect relation between the memorized abnormality (abnormalities) or variation (variations) of use conditions and said error increase; said stored records including: - the error messages relating to an engine and/or the vehicle, which are posted in a vehicle data network and stored in an engine control processing unit and/or the vehicle control processing unit; and - the detection of an exceedance of a minimum tyre pressure threshold, a disconnection and reconnection of the trailer, an activation of a power take-off. - (Step 1) a first acquisition of at least. a first set of input parameters related to environmental and mission conditions,. a second set of input parameters related to vehicle specifications and. a third set of input parameters related to a vehicle driving style; said vehicle "driving style" meaning those conditions under which the vehicle is used by the driver, both negative and positive accelerations applied to the vehicle, the frequency of use of the brakes, the use of the cruise control, the optional use of the Stop & Start;. a first set of input parameters related to environmental and mission conditions,. a second set of input parameters related to vehicle specifications and. a third set of input parameters related to a vehicle driving style; - (Step 2) estimating a first expected fuel consumption in time, based on said first, second and third set of input parameters, - (Step 3) a second acquisition of a measured fuel consumption in time, - (Step 4) comparing, preferably in real time, said expected fuel consumption with said measured fuel consumption, thereby obtaining an error value in time, - (Step 5) checking: whether said error value increases abruptly (5 = Yes), if so - (Step 6) activating a flag indicating an extra-fuel consumption condition; said method further comprising performing the following steps in succession when said flag is active: - (Step 7) acquiring a fourth set of input parameters related to stored records relating to events concerning abnormalities and variations in the operating conditions of the engine and/or vehicle, - (Step 8) checking: whether at least one record is present, then - (Step 9) the instant of memorization of said record (of each of said records) is compared with said activation of said first flag so as to identify a cause-effect relation between the memorized abnormality (abnormalities) or variation (variations) of use conditions and said error increase; - the error messages relating to an engine and/or the vehicle, which are posted in a vehicle data network and stored in an engine control processing unit and/or the vehicle control processing unit; and - the detection of an exceedance of a minimum tyre pressure threshold, a disconnection and reconnection of the trailer, an activation of a power take-off.