(1) A tax is hereby imposed upon the use of motor fuel upon highways of this State by commercial motor vehicles. The tax shall be comprised of 2 parts. Part (a) shall be at the rate established by Section 2 of this Act, as heretofore or hereafter amended. Part (b) shall be at the rate established by subsection (2) of this Section as now or hereafter amended.
     (2) A rate shall be established by the Department as of January 1 of each year using the average “selling price”, as defined in the Retailers’ Occupation Tax Act, per gallon of motor fuel sold in this State during the previous 12 months and multiplying it by 6 1/4% to determine the cents per gallon rate. For the period beginning on July 1, 2000 and through December 31, 2000, the Department shall establish a rate using the average “selling price”, as defined in the Retailers’ Occupation Tax Act, per gallon of motor fuel sold in this State during calendar year 1999 and multiplying it by 1.25% to determine the cents per gallon rate.