Fairfax County Board Chairman John F. Herrity charged yesterday that a newly imposed Northern Virginia gasoline tax is cheating motorists in his county, transforming Fairfax into a "Santa Claus for the region."

Herrity and two other officials representing the county -- Supervisor Nancy K. Falck and State Del. Vincent F. Callahan Jr. -- attacked the tax's allocation formular and promised to seek a revision of it from the Virginia General Assembly next year.

Herrity's position, along with his support from Callahan, a senior Republican legislator, is certain to make division of the tax a hot issue during the legislative session.

The issue suupposedly was settled Thursday when the Northern Virginia Transportation Commission, which includes Fairfax representatives, approved the allocation formula. But Herrity, Falck and Callahan are not on the commission.

The regional tax, currently 2 percent of the cost of each gallon of gasoline, is to be used by Northern Virginia localities to help pay their rapidly rising Metro bus and rail costs. Current estimates are that the tax will raise $11 million a year at the current rate. Because Fairfax has proportionately more service stations with heavier traffice than the close-in suburbs of Arlington and Alexandria, it collects more revenue than it gets back. All of the Fairfax Supervisors were aware of this last winter, but they accepted the allocation formula in order to get the tax approved by the General Assembly.

Herrity opposed the formula then and yesterday he indicated his opposition has, if anything, become stronger. "I'd rather have no tax than an unfair one," he said after his press conference yesterday.

Some of Herrity's colleagues on the board reacted angrily to his position. "I really think that was improper," said board vice chairman Martha V. Pennino (D-Centrevill). "The chairman of the Board of Supervisors is supposed to be representing all the members .. . It's dirty pool."

Two other supervisors -- Marie B. Tavesky (R-Springfield) and Joseph Alexander (D-Lee) -- both said they were fearful that the drive to change the allocation formula could be counterproductive and possibly provoke the General Assembly into scrapping the tax altogether.

Both Traveskly and Alexander voted for the critized allocation formula at the transportation commission's Thursday meeting, as did Supervisor Sandra K. Duckworth (D-Mount Vernon). The other two Fairfax representatives on the commission, Pennino and Supervisor James M. Scott (D-Providence) were not present, but both are on record supporting the allocation formula as the best deal that the county could get from a General Assembly that is not very sympathetic to Northern Virginia causes.

At yesterday's press conference, Callahan said he and another Republican colleague representing Fairfax, Warren E. Barry, will offer legislation at the next session of the General Assembly that would keep every penny raised at Fairfax gas pumps in the county.

The prospects of such legislation passing are uncertain at best. To pass it, the House of Delegates would have to repudiate the original author of the regional gas tax plan -- Del. Warren G. Stambaugh (D-Arlington). "Stambaugh could keep it in committee [Finance]," said one observer. "He has fairly high standing."

Even if the Callahan-Barry plan clleared the House, it could face a tougher battle in the Senate, which showed little sympathy last session for Fairfax's efforts to keep all tax revenues it raised. "I think it's futile," said Sen. Adelard L. Brault (D-Fairfax), who led Senate efforts to get a regional gas tax for Metro.

According to Herrity, Fairfax will collect 61 percent of the gasoline tax, but get only 50 percent of the total revenues.

"It's not all we desire," said Penino, "but we have to recognize we are part of the region. If the region benefits, so do we."