D.C. City Council member Marion Barry yesterday made what he called a "firm, unyielding commitment" to try to obtain a cent reduction in the city's property tax rate for the fiscal year beginning this October.

Considered a likely candidate for mayor or City Council chairman in 1978, Barry said the rate reduction could result from a $12.2 million, two-year financial windfall the city should receive in unexpected property tax revenues.

The windfall stems from recently increased assessments in the value of some 148,000 city properties. City tax assessors expected an increase of about 5 per cent in assessed values, but instead there was an average increase of 2 per cent, Barry said.

The additional increase in assessments means that city will receive $3.7 million more than expected in the current fiscal year and $8.5 million more than expected next year, according to Barry.

As a result, Barry contended, the city should be able to raise the $166 million in anticipated tax revenues with a tax rate of $1.74 per $100 of assesed value instead of the present rate of $1.83.

Barry said the Council should not necessarily be "looked into" the $1.74 rate. "I'm just saying very early and up front that we ought to give the $12.2 million back to the people. Why should we spend money just because we've got it?" he said.

Rising property tax assessments have been a focal point of several Council members' concerns in the past week during which city property tax bills were mailed out and a Superior Court judge blocked one city assessment plan on grounds that it was unfair.

Last week, Council member David A. Clarke (D-one) introduced three bills that he said would more evenly distribute the city's property tax load and thus reduce residential tax rates.

Clarke proposed an end to the exemptions now given by Congress to 30 organizations with substantail real estate holdings in the city, establishment of different tax rates for commercial and residential property owners (both now pay the same) and development of a deterrent tax on speculative real estate sales.

Real estate speculation is regarded by some city officials as one of the major reasons that the assessed values in some neighborhoods are skyrocketing.

Reporters at yesterday's press conference questioned Barry on the political motivations of his lower tax rate announcement. "Some of us down here do things because it's right to do, and we don't worry about the political consequences," Barry responded. "This is strictly an economic and fiscal policy issue."