Joint reports.

(b) Such joint reports shall show the total number of over-the-road miles traveled in this state and the total number of gallons of motor fuel purchased in this state by the reporting carriers. Credits to which the carriers making a joint return are entitled shall not be allowed as credits to any other carrier; but carriers filing joint reports shall permit all carriers engaged in this state in pool operations with them to join in filing joint reports. (c) The vehicles of carriers filing joint reports shall be deemed to have consumed on the average one gallon of motor fuel for each five miles traveled unless evidence discloses that a different amount was consumed.