If the Metro subway system is shortened significantly to save money, Washington area residents will pay more to subsidize public transportation, will drive more miles in their automobiles and thus will consume more energy.

That is a central conclusion of a new analysis from Peat, Marwick, Mitchell & Co. for the regional task force that is restudying the Metro system at the insistence of the federal government.

The analysis also makes clear that the restudy of Metro is really a competition between federal and local governments to see who will spend less.

That is because longer Metro systems will be more expensive to build, but will be built mostly with federal money. Shorter Metro systems will require substitution of more bus service and will be more expensive to operate. Most operating costs not paid by fares come from local taxes.

If one assumes, as the study does, that truncated rail lines would require adding enough buses to provide equivalent public transportation, it is in the interests of local governments to build the longer Metro system. At the same time, it may be in the interest of the federal government to discourage that construction and put its money elsewhere.

"That clearly is what seems to be coming from this study," said George Wickstrom of the Metropolitan Washington Council of Governments and staff director for the study.

"The next question," Wickstrom said, "is whether you want to provide that high a level of public transportation." That question is among those that will have to be answeredin the months to come as the local governments and the federal Department of Transportation work toward a solution of the Metro puzzle.

The PMM & Co, study, presented to the task force meeting yesterday, also made these key points:

The Pentagon station on the subway already has more people using it than the station was designed to handle. If the proposed subway line to Franconia is not built -- and that line is considered the most vulnerable in Metro's planned 100-mile system -- major changes would have to be made at the Pentagon.

If, as some have charged, the ridership projections for Metro are overstated by about 20 per cent, there would be an enormously expensive increase in local budget support needed to keep Metro operating. "Metrorail deficits increase markedly with the 20 percent reduction in patronage and revenue," the report said, "since operating costs can be reduced only marginally, while Metrobus services could be more closely tailored to meet demand."

The PMM & Co. study is one of the final steps the regional task force must take before forwarding its report to the federal Urban Mass Transportation Administration for review.

What the study does is compare construction costs, operating costs, ridership, deficits, bus requirements and many other factors for four alternative Metro systems to the 100-mile system presently on regional maps.

Sixty miles of Metro have federal blessing and financing. Four miles are partly funded and unopposed by the federal government. Another five miles -- the Glenmont line from Silver Spring -- are under final design but do not have federal approval for construction.

That leaves about 30 miles of unresolved Metro. The four systems under study include various cutbacks or reroutings within those 30 miles. They were selected at an earlier meeting of the task force.

Construction costs of the systems beyond 60 miles range from $2.2 billion to $2.8 billion. The shortest system is the cheapest. But the most expensive system -- essentially the Metro as planned today but with a terminal at Tysons Corner instead of Vienna --would attract 414.9 million riders annually -- or 10 million more than the shortest system.

Further, that system and the systems as presently planned (with two minor variations) are cheaper in terms of annual deficit for local governments by $5.3 million and $7.7 million, respectively. All deficit and revenue projections assume fares will keep pace with inflation. That has proven to be a heroic assumption in the present Metro financing picture.

All the proposed systems -- even the shortest -- exceed a total system construction cost ceiling of about $5 billion that can be implied from statements by federal officials.

The Pentagon station problem is at the crux of a Fairfax County -- Alexandria City dispute. Alexandria's City Council, in a series of votes, has opposed extending Metro beyond the Van Dorn station in Alexandria to Franconia in Fairfax County near Springfield Mall. Several influential County Board members want the Franconia station.

The Alexandria position is that the Springfield-Franconia area can best be served by the Shirley Highway busway. Under Metro's present policies, those buses would end their routes at the Pentagon station, and dump thousands of passengers into an "inhumane situation," as Alexandria Councilman Robert Calhoun called it yesterday.

Calhoun and other rush-hour riders know that the escalators, platforms and faregates at the Pentagon cannot handle the crush today, much less in 1990.

"I don't think buses themselves are the problem," said PMM & Co. consultant Jeffrey Bruggemann. "It's the people who are on the busses."

"No easy solution appears obvious," the PMM & Co. report said. "Alternatives might include bypass buses into downtown Washington, diversion of buses to (nearby) Pentagon City, or similar measures, in addition to any enlargement of the (Pentagon) station that would obviously be quite expensive."

Metro planner Mark Akins said yesterday that when the Pentagon station was planned in the late 1960s, there were no Shirley Highway bus lanes and the impact of that major transportation addition simply was not seen.