Alternatively to the methods otherwise set forth in this act, upon application to the director on forms prescribed by the director, the tax may be computed on a mileage basis by an LP-gas user. For the purpose of determining the amount of the tax computed on such basis, the number of gallons of LP-gas used on the highways of this state shall be determined by using the following schedule for calculating the number of miles per gallon which a gallon of LP-gas would propel motor vehicles:

L. 1973, ch. 402, § 4; July 1.