PRESIDENT CARTER wants yet another way to promote fuel conservation, the Council of Governments has an idea for him to pursue. By getting rid of a government subsidy to one class of commuters, Mr. Carter could eliminate some inequities, encourage more use of mass transit, fight air pollution and bring in more federal revenues. The plan is quite simple: Just eliminate free and subsidized parking for federal employees.

Even before energy-saving came into style, federal parking subsidies were a sore point in transportation planning for the national capital region. Federal workers account for about half or the roughly 140,000 commuters' cars that come every day into the central employment area - downtown Washington, Capitol Hill, the Pentagon and nearby Arlington. About 41,000 of those cars are parked in spaces provided by federal agencies and Congress. Three-fourths of those spaces are free. The rest cost from $5 to $20 a month, appreciably less than commercial rates.

What's wrong with this picture? To start with, there is the obvious inequity of providing free or cheap parking to some federal workers while leaving others - not to mention non-federal workers - to use mass transit, scramble for spaces on the streets or park in commercial facilities at rates that may exceed $50 a month.

Beyond that, the subsidies undermine every governmental effort to get area commuters out of their cars and onto public transportation. Federal officials say this is less true than it used to be because several agencies now assign their parking spaces only or mainly to car pools. The General Services Administration claims that 2,700 GSA-controlled spaces accommodate 11,780 people, for an impressive average of four to five people per car. If that is so, charging market rates would not impose great hardships on people who pool; $40 a month divided among four people would be just $10 apiece. If, on the other hand, those figures include a number of phantom car-poolers, as rumors suggest, raising the rates might encourage more drivers to round up real riders to share the cost.

Some officials claim that subsidized parking is needed because mass-transit service is inadequate. With Metro reaching the suburbs this year, that argument can now be turned around: Fairer parking prices should be imposed to encourage more federal employees to leave their cars at home and use the combined bus-rail system instead. Some transit promoters at COG would like to go one step further by investing the added federal-parking revenues in better regional transit. That seems to us to involve new questions of equity and subsidy that would be better left for another day. Right now, it would be a breakthrough if federal agencies, including Congress, would show some of that initiative and spirit of self-sacrific the President has talked about and raise the cost of driving themselves to work.