Acting on behalf of the nation's mayors, New York Mayor David N. Dinkins made a final plea to the Bush administration yesterday to adjust 1990 census totals to compensate for people missed in the decennial headcount.

In a meeting with Commerce Secretary Robert A. Mosbacher, Dinkins reiterated his concern that the census has missed millions of Americans, many of them low-income minorities living in big cities. In his city alone, census work conducted over the past months has missed around 800,000 residents, Dinkins said.

Dinkins, representing the U.S. Conference of Mayors, was accompanied by William Althaus, mayor of York, Pa. Althaus contends that as many as 6 percent of the 50,000 residents of his town were not counted.

"We are very, very concerned," Dinkins told reporters after the meeting. Without an adjustment, which would add or subtract population based on a statistical model, "it could cost us a billion dollars over the next 10 years."

Commerce officials, who oversee the Census Bureau, said yesterday they remain open to an adjustment, but a decision will not be made until next summer.

The issue is at the heart of what has become a controversial census, with Democratic members of Congress and city officials complaining bitterly that the process was seriously flawed, with millions of residents missed as a result.

If an adjustment were ordered, it would be the first time in history and would change dramatically the way the 200-year-old headcount was conducted.

While Census Bureau Director Barbara E. Bryant has said she has not decided whether the numbers should be adjusted, she has repeatedly disputed criticism of the 1990 census. "We feel we've done a very, very thorough census," she said yesterday. "We're quite confident we have really touched bases with every housing unit."

Under a legal agreement stemming from a lawsuit filed by New York City and several other jurisdictions, Mosbacher must consider an adjustment, hear the recommendations of a panel of experts and announce his decision by July 15, 1991.

In a separate meeting with reporters, Bryant said city officials probably will be disappointed when they are informed over the coming weeks of the bureau's population totals for their jurisdictions. While most totals have risen since preliminary figures were released in August, the original counts conducted by census workers were closer to the final figures than were those submitted by local governments challenging the bureau's findings.

The challenges, filed by about 7,600 local governments, uncovered "geocoding errors," in which census workers assigned housing units to the wrong blocks. While correcting those errors will not change population totals, they will be important when political districts are redrawn over the coming year.

Bryant said hundreds of thousands of households and individuals have been added to the early census numbers since they were released in August, but she would not specify a total population figure for the nation. The official tally will be released by Dec. 31.

Census workers have rechecked about 20 percent of the nation's 106 million households, according to Bryant. These additional programs have found more than 400,000 persons on parole or probation lists not found by the spring headcount, for example. And at least 165,800 persons were found through a "Were You Counted" campaign.

But even if these late programs yield 2 million or 3 million persons, the total population figure will fall below the bureau's estimates, which range from 250 million to 253 million. While census officials say the discrepancy can reflect problems in the estimates as well as the census, critics said the gap is further evidence of an undercount.

TerriAnn Lowenthal, an aide to Rep. Tom Sawyer (D), chairman of the House subcommittee on census and population, said the figures released by Bryant yesterday bear out predictions made by Sawyer in November that the final tally will fall short of the Census Bureau's estimates.

"While we won't know for several months the exact extent of any undercount, it may be substantial," she said.