Okla. Stat. tit. 68 § 2203
It is hereby declared to be the intention of the Legislature that the tax herein imposed be not greater than the amount of tax such freight line companies, equipment companies, and mercantile companies would pay if their cars were taxed on an ad valorem basis, including any value inuring to such cars by reason of being a part of a going concern.
The Oklahoma Tax Commission upon the complaint of any person who claims he is taxed too great a rate hereunder, shall take testimony to determine whether the taxes herein imposed are greater than the general ad valorem tax for all purposes would be on such freight cars, if taxed on an ad valorem basis. The Commission shall have the power and it shall be its duty to lower the rate herein imposed to conform to the facts disclosed at said hearing.
In order to determine the amount of tax such companies would pay, said Commission may value all cars of any company as a unit and allocate to Oklahoma that proportion of the total value which the Oklahoma car mileage bears to the total car mileage of the cars of any such company during the twelve-month period ending on December 31 of any year, and may then apply to such value so ascertained the average ad valorem tax rate applied to property throughout the state for that calendar year.
Okla. Stat. tit. 68, § 2203