Monthly pension benefits of the average couple equal about three-fifths of pre-retirement wages if the couple is entitled to both Social Security and some other pension, according to a new government study.

But if the couple gets only Social Security, without any other private or public pension, monthly benefits are only about half pre-retirement earnings.

These figures have large implications for a developing dispute over whether Social Security benefits are too high. The figures indicate that pensions for the average couple fall far short of replacing wages lost as a result of retirement.

For example, in 1974 the average pre-retirement earnings of newly retired couples with both Social Security and a non-Social Security pension were $11,315, according to the study, but the typical pension income was only $6,800.

Moreover, the statistics indicate that when the couple gets Social Security only, the amount of wages replaced is lower in practice than the goals set out by Congress for the average husband and wife in retirement.

For the average family, Congress intended the combined husband-wife Social Security pension to replace about two-thirds of former wages. But the figures indicate that in practice, only about half the former wages for such a couple was replaced by the Social Security payments. This is far lower than Congress intended and isn't even as high as the two-thirds goal when a private pension is added.

These are the findings of a new study by Alan Fox of the Social Security Administration, based on retirement histories of couples going on Social Security between 1968 and 1974.

Although every individual's expected pensions must be based on his or her work history, the findings are a rough guide to what anyone can expect to get.

The study showed that nearly half the couples going on the Social Security rolls in 1968-74 received a non-Social Security pension. About 35 percent received a private pension, and 10 percent some form of local government pension.

But most of those getting the second pension were high-income families before retirement. Three-quarters of those with combined family wages of $12,500 a year before retirement got some second pension, but the percentage dropped sharply for those of lower income -- to 36 percent in the $6,000-to-$8,000 category and 17 percent in the $4,000-to-$6,000 range.

These figures reflect that normally only the more profitable, better-paying private businesses can afford to set up private pension systems to supplement Social Security.

The study calculated average annual pre-retirement income by finding combined annual wages for the husband and wife during their three highest years of earnings in the decade before retirement. This included earnings in excess of the Social Security taxable wage-base. Very high-income families were excluded to prevent skewing the study.

Then, Social Security pensions plus pensions from private employers or state and local governments were added for the family.

Those figures were used to determine what percentage of pre-retirement gross income was received in pensions.

The study found that in 1974, families with only Social Security (the husband's and wife's benefits combined) got a pension equal to 49 percent of pre-retirement wages, on the average.

Those receiving Social Security plus another pension got 62 percent.