For the purposes of making payment of taxes and filing reports pursuant to this article, the year is divided into four quarters of three consecutive months each. The first quarter of the year shall consist of the months of January, February, and March. The road tax shall be paid by each motor carrier to the commissioner on or before the last day of the month immediately following the quarter with respect to which tax liability under this article accrues and shall be calculated upon the amount of motor fuel used by the motor carrier in its operations within this state during the quarter ending on the last day of the preceding month. The amount of motor fuel used in the operations of any motor carrier within this state shall be the proportion of the total amount of motor fuel used in its entire operations within and outside this state which the total number of miles traveled within this state bears to the total number of miles traveled within and outside this state.