Say goodbye to the motor pool.

Beginning Oct. 1, the General Services Administration plans to eliminate its fleet of 4,000 vehicles that federal officials can call on when they need a car. Instead workers will have to turn to one of the commercial car rental companies that have signed contracts with GSA.

The so-called "dispatch" vehicles, primarily sedans and station wagons, are now available for short-term use from 140 motor pools around the country. Under the new procedure, employes will have make their own arrangements to rent a vehicle from one of the contractors for rates averaging $11 a day.

William F. Marshall, director of GSA's interagency motor pool, said the change "is part of a new program designed to manage the government's automotive needs."

Marshall said the agency expects to save $2.9 million by eliminating the service. It now costs GSA about $24 million a year to keep the bulk of the 82,115 cars in its control repaired and in service. About 57,000 are now assigned to government agencies on a permanent basis and will continue to be.

Currently, the motor pool cars are checked out about 150,000 times a year. Since a car is not always available, GSA has been forced to spend about $8 million a year to rent 180,000 cars on a spot basis.

But Marshall said he expects that only about 270,000 cars will be rented, because once employes lose the convenience of the motor pool, a "significant number of people will use mass transit and personally owned vehicles instead . . . and will be reimbursed." The government reimburses employes 20 cents a mile when they use their cars on government business.

The rental rates that the government now pays range from a high of $21.60 for a Budget Rent-a-Car in Juneau, Alaska, to a low of $9.12 for a Thrifty Rent-a-Car in Lawton, Okla. In Washington, American International Rent-A-Car provides the government with vehicles for $12.05 a day.

GSA also plans to turn over the cars it still has more often. Government cars are now replaced every six years or 60,000 miles, whichever comes first; GSA now wants to buy new cars with a three-year warranty, and resell them at the end of three years. The 4,000 on-call vehicles will be used during the transition period.

To help maintain the cars, GSA plans to increase the number of repair contracts it has with garages and auto dealerships and set up 11 regional telephone numbers with government mechanics to monitor the work.

"The central maintenance concept is where you put an automotive inspector at the other end of the telephone," Marshall said. "Whenever someone calls in and gives the tag number, the inspector can bring up the history of the vehicle on the computer and help diagnose the problem. He can see if the car is still under warranty and, by computer, see where the nearest certified repair shop is."

The changes will mean the agency's work force will drop by an estimated 325 positions. "I'm sure some regions will have RIFs," said Lester L. Mitchell, assistant administrator of GSA for federal supply and services.