The valuation of such private cars apportioned to this State shall be determined by the department. The department shall determine the average number of each class of private cars physically present in the State in the year immediately preceding the year in which the tax is imposed upon the basis of car mileage, car days, or such other data as would tend to establish this average. The department shall multiply the average number so determined by the fair market value for a car of that class and use the product as the basis for the assessment of the property.