A presidential advisory panel used out-of-date figures when it said it had figured out a way to save the government $16 billion in retirement benefits over three years, Comptroller General Charles A. Bowsher told a congressional panel yesterday.

Bowsher, testifying before a House Post Office and Government Operations subcommittee on a report by the President's Private Sector Survey on Cost Control, also said that a recommendation to make government pay comparable to private-sector pay could cost the government several billion dollars -- about as much as the survey said it would save.

The survey, composed of more than 160 businessmen, was appointed by Reagan last year to find ways of saving money by making goverment more efficient. Task forces working under the panel have identified more than $100 billion in potiential savings so far, with the biggest chunk -- more than $35 billion -- coming out of federal pay and benefits.

Bowsher said the General Accounting Office was "concerned about the bases for the savings estimates," particularly the figures used to calculate savings by reducing retirement benefits.

The survey's personnel management task force reported that 29 percent of the government's personnel costs are for retirement benefits, compared to only 14 percent for the private sector. Bowsher cited a 1981 Congressional Research Service report that put the government figure at 24.2 percent and a 1983 Congressional Budget Office report that put the private-sector figure at 22.8 percent.

Bowsher also questioned the presidential panel's statement that Uncle Sam's retirement plan encourages early retirement.

"This is a commonly held perception that is not supported by acutal experience," he said, citing surveys that put the average age of government retirees at 61 and private-sector retirees at 61.8.

A member of the panel, W.R. Grace & Co. executive Felix Larkin, defended the report, telling the House members that it was completed in October, 1982, and used the most recent data then available.

"We do not profess to be more infalliable than Congress and I think it would be astronishing if there were not errors in the reports," Larkin said.