An organization of Virginia service station operators wants to head off a projected doubling of the 2 percent regional gasoline sales tax, contending it shortchanges the statewide highway program while benefiting the Metro transit system.

James Heizer, executive director of the Virginia Gasoline Retailers Association, told a Richmond news conference that motor fuel sales in the five affected Northern Virginia jurisdictions in the first five months of 1981 were down 7.3 percent from the same period last year, compared with a 2.7 percent statewide decline.

A similar downward trend in the 1980 fiscal year resulted in a $2.4 million absolute loss of state highway revenue even though it produced $9.6 million to support Metro, Heizer said. He said many Virginians apparently are buying gasoline cheaper in Maryland or the District of Columbia.

While the financially hard-pressed Northern Virginia dealers would like to see a repeal of the present 2 percent regional tax, which translates to nearly 3 cents a gallon, Heizer acknowledged the chances are slim. "But, at the very least," he said, "I would hope the legislature would take action so the additional 2 percent won't take effect" as scheduled next July 2.

Joseph Alexander, Metro board chairman and a member of the Fairfax County Board of Supervisors, said a cutback of the gasoline sales tax would force the five jurisdictions to levy some other taxes to make up the difference. He said the jurisdictions are counting on the higher gasoline tax next year to produce the "stable and reliable source of revenue" required by federal law to pay the local share of Metro costs.

The tax is levied in Arlington and Fairfax counties and the cities of Alexandria, Fairfax and Falls Church.