President Carter oversees the world's largest and most lavish motor pool, according to the government's own records and gasoline bills.

The federal government has about 450,000 civilian cars, trucks and buses -- one vehicle for every six federal employes -- and some government departments are renting additional cars and vans during this time of energy difficulties.

In the Washington area alone, federal officials -- who asked not to be identified -- estimate some government departments and agencies are spending several hundred thousand dollars each month for VIP-style cars and for multipassenger vans. They are used to haul workers -- often only one or two passengers per trip -- between agencies and often to transport them between nearby buildings of the same agency.

According to recent federal estimates, the civilian side of government now spends nearly $700 million each year on gasoline for the cars, trucks and buses it owns outright. During the 1977 fiscal year agencies used 338 million gallons of gasoline to run their fleets.

Federal officials say that if that amount was translated into crude oil terms, it would come to 17.5 million barrels a year. (During the 1973-74 Arab oil embargo, federal officials say, the nation suffered a shortage of about 1.6 million barrels each day.)

Many federal agencies here lease cars for the special use of officials. Some also operate regular bus services, moving small groups of employes to different agencies for short-time assignments or visits, and then back to their home agency before quitting time. Examples of the government's in-house transportation network uncovered by The Washington Post include:

The Department of Energy, which leases a number of 15-passenger vans for regular runs between buildings. A spot check of those vans showed many of them running between stops with only two or three passengers. Some of the runs were "dead heads" according to the drivers, meaning the trip was made even if there were no passengers.

Army, Navy and Air Force installations use school-bus type vehicles on a regular basis to transport small numbers of workers from building-to-building, often only a few blocks apart.

Cars leased for the government by the General Services Administration, or borrowed from GSA motor pools are regular features around town.

National Park Service cars often ferry workers in relatively remote area worksites to nearby coffee shops for mid-morning, lunch and afternoon breaks.

The Central Intelligence Agency operates a very private bus service, carrying lesser-ranked workers and messengers around town, and to and from an Arlington office to its Langley headquarters.

The U.S. Postal Service uses around 92 million gallons of gasoline a year for its fleet of 123,000 government-owned trucks, jeeps and cars. Costs of operation for maintenance and gasoline run $394 million, and $240 million a year respectively.

Although the Postal Service has one of the largest federal vehicle fleets, many officials believe it has one of the best gas-saving records in government. Most of its cars and trucks are on tight pickup and delivery schedules that minimize unauthorized use or stops. In addition, the USPS is also experimenting with a number of electric vehicles suited for stop-and-go work over 30 to 40 miles.

Washingtons are accustomed to seeing chauffeur-driven federal limousines parked outside restaurants, hotels or other meeting plances. Drivers often keep the motors running summer and winter to keep the cars cool, or warm, for themselves and their VIP bosses.

Some government drivers, records show, made almost as much money in overtime as they do in regular salary. The extra income results from chauffeuring top federal officials to afterwork functions, including parties and embassy receptions, and for pickup and delivery to airports.

According to government records kept by the General Services Administration, the average cost per mile to the federally owned auto fleet now runs in excess of 20 cents for every mile driven. That amount is down from previous years.

Despite White House orders to use smaller, gas-efficient cars, federal agencies are continuing to buy and lease larger models that cost more and have poorer mileage. Treasury and the State Department have most of the government-owned limousines, although some agencies rent them from private firms for top officials.

Most of the passenger cars in the government fleet are listed as "compact class two." They are Novas, Aspens, Skylarks, and even Cadillac Sevilles. Only a handful of "subcompacts" like Chevettes are owned or leased by the government. Although officials say the number of small, gas-saving cars is increasing, many agencies still request moderately large cars for sedan use.