A new General Accounting Office report says that while federal workers have better pension and leave benefits than private-sector employes, pay and benefits overall are superior outside of government because of a 10 to 18 percent pay advantage.

GAO's study, a compilation of previous federal and private reports comparing compensation, also says that raises for private-sector employes over the past eight years have been 21 percent higher than government pay increases for comparable jobs.

The GAO data, prepared for the Senate Governmental Affairs Committee, illustrates the difficulty in trying to compare compensation in industry and government to decide if federal workers are overpaid or underpaid.

GAO points out that:

*The government's Economic Cost Index shows that private sector pay has risen 65 1/2 percent since 1977, while federal pay was up only 44 1/2 percent.

*Pay actions by Presidents Carter and Reagan have produced a gap of 18.2 percent between pay of government workers and the private sector, according to data compiled by the Bureau of Labor Statistics.

*A study done for the House Post Office-Civil Service Committee by a private consultant using different techniques than the BLS said the pay differential was about 10 percent in favor of industry.

*The retirement system covering U.S. workers hired before January 1984 lacks industry benefits -- and risks -- such as profit sharing, stock options and thrift plans. But it is still more generous and costly because it allows federal workers to retire earlier and provides for annual raises to keep pace with inflation.

*Federal workers, who earn either 13, 20 or 26 days of annual leave a year (depending on length of service) get more leave than private employes -- whose vacation time ranges from about eight days after one year of service to 20 1/2 days after 20 years.

*Although the average federal worker retires at age 61 after 29 years service, employes have the option of retiring on reduced benefits at age 55 after 30 years. Private employes, GAO said, take benefit cuts ranging from 2 to 4 percent a year for each year they are under age 62.

*Nine of 10 private companies surveyed provided better health insurance, at less cost to employes, than the GAO's study . . . says that raises for private-sector employes over the past eight years have been 21 percent higher than government pay increases . . . government, which pays an average of 64 percent of its employes' premiums.

GAO suggested that Congress may want to consider legislation that would change the government's salary-setting system from a straight matchup with private industry to a system that compares the value of total compensation. That would also include retirement, leave, insurance and other benefits.