((a)) ** Additional provisions applicable** The provisions of this section shall apply, in addition to the other provisions of this part, in the case of covered fleet vehicles owned or operated by an agency, department, or instrumentality of the United States, except as otherwise provided in subsection (e).

((b)) ** Cost of vehicles to Federal agency** Notwithstanding the provisions of sections 601–611 of title 40, the Administrator of General Services shall not include the incremental costs of clean-fuel vehicles in the amount to be reimbursed by Federal agencies if the Administrator of General Services determines that appropriations provided pursuant to this paragraph are sufficient to provide for the incremental cost of such vehicles over the cost of comparable conventional vehicles.

((c)) ** Limitations on appropriations** Funds appropriated pursuant to the authorization under this paragraph shall be applicable only—

((1)) to the portion of the cost of acquisition, maintenance and operation of vehicles acquired under this subparagraph which exceeds the cost of acquisition, maintenance and operation of comparable conventional vehicles;

((2)) to the portion of the costs of fuel storage and dispensing equipment attributable to such vehicles which exceeds the costs for such purposes required for conventional vehicles; and

((3)) to the portion of the costs of acquisition of clean-fuel vehicles which represents a reduction in revenue from the disposal of such vehicles as compared to revenue resulting from the disposal of comparable conventional vehicles.

((d)) ** Vehicle costs** The incremental cost of vehicles acquired under this part over the cost of comparable conventional vehicles shall not be applied to any calculation with respect to a limitation under law on the maximum cost of individual vehicles which may be required by the United States.

((e)) ** Exemptions** The requirements of this part shall not apply to vehicles with respect to which the Secretary of Defense has certified to the Administrator that an exemption is needed based on national security consideration.

((f)) ** Acquisition requirement** Federal agencies, to the extent practicable, shall obtain clean-fuel vehicles from original equipment manufacturers.

((g)) ** Authorization of appropriations** There are authorized to be appropriated such sums as may be required to carry out the provisions of this section: , That such sums as are appropriated for the Administrator of General Services pursuant to the authorization under this section shall be added to the General Supply Fund established in .section 321 of title 40Provided