Intercity buses operated in interstate commerce; tax.

All intercity motor buses owned or operated by residents or nonresidents of this state in interstate commerce or combined interstate and intrastate commerce as a result of which operation such motor buses operate both within and without this state under the authority of the United States Department of Transportation, are subject to motor vehicle license taxes on a basis commensurate with the use of Florida roads. The department shall require the registration in this state of that percentage of intercity motor buses operating in interstate commerce or combined interstate-intrastate commerce, into or through this state, which the actual mileage operated in this state bears to the total mileage all such intercity motor buses are operated both within and without this state. Such percentage figure, so determined, is the “Florida mileage factor.” In determining the state license tax to be paid on the buses actually operated in this state under the foregoing method, the department shall first compute the amount that the state license tax would be if all of such buses were in fact subject to such tax, and then apply to that amount the Florida mileage factor.