Americans may be spending $215 million a year more than they should for medical services because of arrangements some doctors have with hospitals, Ralph Nader's health experts said yesterday.

The Health Research Group said in an unpublished study made for the Department of Health, Education and Welfare shows the extra costs result when doctors get a percentage of the business at hospitals or a free for service, instead of a straight salary.

Those doctors, it said, tend to promote high volume and perhaps unnecessary use of their services in order to increase their income.

Radiologists based at hospitals make $57,000 more per year per doctor than their colleagues who are paid straight salaries by hospitals, the group said, while the difference in earnings for pathologists is $75,200 per year per doctor.

"We calculate that if all hospital-based radiologists, pathologists and anesthesiologists in the United States had been paid salaries at the average rates shown in the study during fiscal 1975, instead of having fee-for-service or percentage compensation arrangements, the aggregate savings for American taxpayers and consumers for the year would have been about $215 million," the group said.

[A health cost containment bill introduced by Sen. Herman Talmadge (D-Ga.) would end percentage arrangements for hospital-based doctors, but continue to let them bill patients directly.]

The Nader group released a letter to HEW Secretary Joseph A. Califano Jr. urging him to amend the administration's proposed hospital containment cost legislation to require all hospitals receiving Medicare and Medicaid funds to pay hospital-based doctors on a salary basis only.

Unless that is done, the letter said, "What is perhaps the fastest increasing component of hospital costs would thereby escape regulation."

The study, it said, shows that the "present, largely unregulated fee-for-service system creates incentives for physicians to fill their pockets by ordering excessive hospital tests and services.