No county or city shall have the right to impose a tax upon the gross revenues derived by a metropolitan municipal corporation from the operation of a metropolitan sewage disposal, water supply, garbage disposal or public transportation system.

A metropolitan municipal corporation may credit or offset against the amount of any tax which is levied by the state during any calendar year upon the gross revenues derived by such metropolitan municipal corporation from the performance of any authorized function, the amount of any expenditures made from such gross revenues by such metropolitan municipal corporation during the same calendar year or any year prior to May 21, 1971 in planning for or performing the function of metropolitan public transportation and including interest on any moneys advanced for such purpose from other funds and to the extent of such credit a metropolitan municipal corporation may expend such revenues for such purposes.

A metropolitan municipal corporation authorized to perform the function of metropolitan public transportation and engaged in the operation of an urban passenger transportation system shall receive a refund of the amount of the motor vehicle fuel tax levied by the state and paid on each gallon of motor vehicle fuel used, whether such vehicle fuel tax has been paid either directly to the vendor from whom the motor vehicle fuel was purchased or indirectly by adding the amount of such tax to the price of such fuel: PROVIDED, That no refunds authorized by this section shall be granted on fuel used by any urban transportation vehicle on any trip where any portion of said trip is more than six road miles beyond the corporate limits of the metropolitan municipal corporation in which said trip originated.