“ The Administrator of General Services shall also promulgate rules which will ensure that each class of nonpassenger automobiles acquired by all Executive agencies in each fiscal year achieves a fleet average fuel economy that is not less than the average fuel economy standard for such class, established pursuant to Section 502 (b) of the Motor Vehicle Information and Cost Savings Act, as amended (89 Stat. 903, 15 U.S.C. 2002 (b) ) , for the model year which includes January 1 of such fiscal year. ”

“ Such rules shall not apply to nonpassenger automobiles intended for use in combat-related missions for the Armed Forces or intended for use in law enforcement work or emergency rescue work. The Administrator may provide for granting exceptions for individual nonpassenger automobiles or categories of nonpassenger automobiles as he determines to be appropriate in terms of energy conservation, economy, efficiency, or service. ”