A correlogram is initially prepared which represents a relation between the gross volume of all tries and the maximum gross weight of a motor vehicle. The maximum gross weight of the vehicle is one of important factors for determining a charge for a vehicle to be run on the toll road. In the method of this invention the outer diameter and rim diameter of the respective wheels of the vehicle are found through the processing of the side profile image of the vehicle as obtained by a TV camera. According to this invention the total volume of all the tires is calculated from the outer diameter, rim diameter, width and total number of the wheels. In this way, the maximum gross weight of the vehicle is calculated based on the correlogram.