Transportation Secretary Drew Lewis said yesterday the Reagan administration is "firmly committed" to a 75-mile Metro subway system that would include a major inner-city section of the long-threatened Green Line.

Furthermore, Lewis said, "I anticipate that the additional miles [of the planned 101-mile system] will be completed. We are waiting on the local governments to see it their funds will be available for that."

Lewis' commitment to 75 miles came during a briefing for the press on the Department of Transportation's budget and is the most postive statement any Reagan administration official has made concerning Washington Metro. By mentioning 75 miles, Lewis is clearly going beyond the 62 miles of subway that are already in operation or funded virtually to completion.

However, other DOT officials said, Lewis is proposing that federal funds for Metro construction be limited to $275 million a year, the same amount the Carter administration had approved in recent years. Carter had agreed to a new level of $350 million, however, so $275 million represents a cut in what Metro had hoped to receive and will slow the pace of construction.

A 75-mile system would include the 37 miles already in operation plus these segments:

The Red Line extension from Dupont Circle north to Shady Grove in Montgomery County. That would open in two phases, from Dupoint Circle to Van Ness Center in April or May of 1982 and from Van Ness Center to Shady Grove late in 1983.

The Blue Line extension from National Airport south to Huntington in Fairfax County, scheduled to open late in 1982.

The Yellow Line crossing of the Potomac River at the 14th Street Bridge, which would provide a direct connection between the Pentagon, Southwest Washington and Gallery Place. That would open late in 1982.

The Orange Line extension from Ballston to Vienna in Fairfax County, to open in mid to late 1986.

An inner-city Green Line from Anacostia through Southeast-Southwest Washington, north under Seventh Street to U Street NW, and west under U Street to 14th Street NW. The opening for that segment has not been determined, but sometime in 1987 would be the earliest likely date.

A Red Line extension from Silver Spring north to Forest Glen, which would open early in 1987 if Forest Glen is acceptable to Montgomery County as an interim terminal. There is some question on that latter point.

Metro General Manager Richard S. Page welcomed Lewis' statements, calling them "the first clear message of support for Metro from the Reagan administration. "You can't ask for more than that right now," he said. However, Page said, "We are going to continue to push for $350 million," an annual federal funding level that would permit Metro to do more things in a shorter period of time.

Another important point was cleared up yesterday. DOT's projections show that some of the $1.7 billion authorized specifically for Metro construction by the last Congress will definitely be made available. Some Reagan administration officials had opposed use of that special Metro authorization and had argued that Metro should be treated like every other subway construction program in the country and forced to compete for limited funds from a common pot.