The Army defended its new M1 main battle tanks yesterday but estimated that they will run up $27 billion in "support costs" over their predicted 20-year lives, increasing the expected price tag to $47 billion.

The estimate threw into sharp relief an often obscure aspect of the full costs of the 47 major weapons systems now being acquired, Sen. William Proxmire (D-Wis.) said in an interview. Congress should require the Pentagon to provide support-costs estimates for all the systems, he added.

He elicited the $27 billion figure at a hearing of his Joint Economic subcommittee after the Army acknowledged some problems with the M1, rejected criticisms by the General Accounting Office, and went on to acclaim it as "the best tank in the world today" and "an absolute winner."

Support costs -- normally left unmentioned by the armed services when they seek congressional approval of new weapons systems -- are all the money needed to operate a system after it and initial spare parts have been procured, including fuel, maintenance and personnel.

The $27 billion support estimate for the planned total of 7,058 M1s was termed "very, very, low" by subcommittee counsel Richard F. Kaufman. "The rule of thumb is that support costs will be 70 percent to 90 percent of life-cycle costs," as compared with the Army's estimate of 57 percent for the M1, he told a reporter.

GAO officials, testifying Tuesday, based their criticisms of the M1 mainly on a tentative analysis of just-completed nine-month tests at Fort Hood, Tex., and Fort Knox, Ky., and of continuing, 60-percent completed tests at Aberdeen Proving Ground Center in Maryland.

They said the average number of miles the M1s traveled "before they had to stop for unscheduled maintenance" was 30 at Aberdeen (where three tanks had clocked an average of 3,661 miles each), 32 at Fort Knox (four averaging 3,506 miles) and 89 at Fort Hood (six averaging 284 miles.)

Maj. Gen. Richard Lawrence, commanding general of the 1st Cavalry Division at Fort Hood, testified that "I cannot reconcile" the 30- and 32-mile figures with his own experience, adding that his troops find M1s "easier to maintain" than the M60s the new tanks will replace.

Maj. Gen. Duard D. Hall, program manager for the M1, said it meets "the most exacting" tests ever demanded by the Army, provides "unparalleled protection" of crewmen, and has a unique "excellent probability of a first-round hit and kill" while moving at relatively high speed. He admitted some faults, including a track life of half the 2,000-mile goal, and a major shortfall from the goal of 4,000 miles without a failure of its power train, which includes the 1,500-horsepower turbine engine, transmission and final drive.