Under regulations prescribed under
section 5707 of this title, an employee who is engaged on official business for the Government is entitled to a rate per mile established by the Administrator of General Services, instead of the actual expenses of transportation, for the use of a privately owned automobile when that mode of transportation is authorized or approved as more advantageous to the Government. In any year in which the Internal Revenue Service establishes a single standard mileage rate for optional use by taxpayers in computing the deductible costs of operating their automobiles for business purposes, the rate per mile shall be the single standard mileage rate established by the Internal Revenue Service.