(a) As used in this Code section, the term:

(1) "Georgia fleet mileage ratio" means a fraction, the numerator of which is the total miles driven in Georgia by all commercial vehicles registered in Georgia under the International Registration Plan pursuant to Code Section 40-2-88, and the denominator of which is the total miles driven within and without Georgia by such commercial vehicles.

(2) "Gross capital cost" means the freight on board, delivered cost of a commercial vehicle to the purchaser of such commercial vehicle but shall not include any excise or use taxes paid as a part of such purchase.

(b) The valuation of a commercial vehicle, trailer, or semitrailer for ad valorem tax purposes shall be determined as follows:

(1) The gross capital cost of a commercial vehicle, trailer, or semitrailer shall be multiplied by a percentage factor representing the remainder of such vehicle's value after depreciation according to a depreciation schedule which the commissioner shall annually prepare and distribute to each of the tax collectors and tax commissioners. Except as provided in paragraph (2) of this subsection, the resulting value of such commercial vehicle, trailer, or semitrailer shall be assessed at the rate of 40 percent of such value for ad valorem tax purposes in this state; or

(2) For a trailer, a semitrailer, or a commercial vehicle which is not registered in Georgia under the International Registration Plan pursuant to Code Section 40-2-88, the assessment calculated under paragraph (1) of this subsection shall be multiplied by the Georgia fleet mileage ratio. The resulting apportioned value shall be the Georgia assessed value of the commercial vehicle, trailer, or semitrailer for ad valorem tax purposes in this state.