The Metro board yesterday formally proposed raising base fares on buses and trains to 75 cents on April 2. Mileage rates that determine many peak-hour rail fares would go up, while most bus-zone fees would remain the same.

The fare package, which the board could alter following public hearings next month, would generate about $14 million in new revenues for the financially strapped transit system and raise the average cost of a ride by about 15 percent.

Bus-to-bus transfers, now free, would cost 5 cents under the proposal.

In other action, the board approved an experiment to withdraw cars from the Red, Blue and Orange lines in January to enlarge the reserve fleet and possibly divert cars to open the Yellow Line to National Airport at an undetermined date. It also approved educating rail passengers on how to escape from cars in emergencies--information now denied to riders.

In addition, the board authorized soliciting bids for a contractor to build tunnels running 2,000 feet north from the Gallery Place station at a cost of about $35 million. That contract will mark the first major construction work on the long-delayed, "inner-city" Green Line since the late 1970s.

The proposed fare increase, which would be implemented 16 months after the last increase in December 1981, comes as Metro's operating deficit is expected to reach about $180 million this fiscal year, most of it to be covered by local and state governments.

"Jurisdictions are indicating that there must be an increase in fares in order to relieve the pressure on their general funds," said D.C. City Council member Jerry A. Moore, chairman of the Metro board.

A fare increase would be expected to heighten Metro's long-standing ridership problems. But Metro officials predict that patronage will edge up slightly in the year beginning next July despite an April fare rise, if new rail stations open and the system's reliability improves.

The proposed package would raise the current 65-cent base fare for trains to 75 cents. During peak hours, riders would pay another 15 cents per mile after the first three miles, up from the current 13 cents. The board rejected proposals from the District and Virginia to charge mileage rates for off-peak rail rides.

The new formula would raise the peak fare between Metro Center and New Carrollton from $1.60 to $1.85. A Silver Spring-Metro Center peak-fare ride would rise from $1.25 to $1.45 and a Ballston-Metro Center ride from 90 cents to $1.05.

Base fares on Metrobuses would rise a dime to 75 cents, except in the District during off-peak hours, when the fare would be 70 cents.

The package also specifies that:

The Potomac crossing fee would rise to 75 cents. Peak-hour surcharges for the D.C.-Maryland line, the Maryland Zones 1 and 2 line and the Virginia zone lines would stay the same. The off-peak fee of 35 cents for crossing the D.C.-Maryland line also would be unchanged.

* Fares on D.C.'s Metro-Mini buses would rise from 50 cents to 60 cents.

* Elderly and handicapped riders' fares would rise to a maximum of 75 cents. Base bus fares for the elderly would be 30 cents in the District and 35 cents in the suburbs.

* Riders holding rail transfers would pay 50 cents, plus any crossing charges, on boarding buses in Virginia and Maryland -- up from 40 cents plus charges. Rail transfers would continue to be good for the full base fare in the District.

Flash pass prices would rise.

In March the board is to set opening dates for the Yellow Line between National Airport and Gallery Place, the track between National Airport and Huntington, and the Red Line between Van Ness and Shady Grove.

The board yesterday awarded a contract to build an $11-million central maintenance facility in Alexandria, after allaying the concerns of D.C. board members who opposed the Alexandria site. Metro officials assured them that special efforts will be made to recruit District residents for Metro jobs.