For Northeast Washington Exxon dealer John T. McQueeney, the city's new 6 percent gasoline sales tax is proving a business nightmare -- even though the controversial tax does not go into effect until next month.

McQueeney said yesterday that he has been notified by several large commercial customers that they will stop buying gasoline from him and get their fuel at lower prices in Maryland, where there is no gasoline sales tax.

The tax measure, part of a package designed to help the city ease its financial problems, was signed into law yesterday by Mayor Marion Barry and is slated to go into effect on Aug. 1.

When it was enacted under emergency provisions last month, the legislation came under immediate attack by service station representatives and dealers because it would put them at a competitive disadvantage with stations in the Maryland suburbs.

Virginia was not considered a problem because the state raised its gasoline sales tax in the Washington suburbs about 4 1/2 cents a gallon as of July 1.

C. J. Stuecker, general manager of Mann's Potato Chip Co., said the dealer's fears appeared to be coming true. "It's the bottom line," he said.

Stuecker said he will save at least $420 a month by dropping McQueeney as his suplier and having his 35 delivery trucks driven to stations less than a mile away in Prince Georges' County.

"$400 is $400, and I'm probably one of the last manufacturers in the District and they [city officials] are making it harder to do business all the time," Stuecker said.

Other customers of McQueeney's station, located at the city's busy Northeast gateway intersection of New York Avenue and Bladensburg Road, have given similar notice, McQueeney said. These include a liquor wholesaler, a printing company and a paving contractor.

McQueeney said he will lose up to 40 percent of his current business, reducing sales as much as $13,000 a month.

John Pudrier, assistant production manager of Stephenson Printers, confirmed that his company will shift to Maryland its purchases of 70 gallons of gasoline each day. An official of Beitzell & Co., a liquor wholesaler that McQueeney said was taking similar action, declined comment.

The new sales tax will add about 8 cents a gallon to the price of gasoline that now retails at about $1.25.

Among other provisions, the seven-part tax measure raises the city's general sales tax rate from 5 percent to 6 percent, extends the coverage of the sales tax to some now-exempt items and doubles the 1 percent tax on real estate transfers.

Of all the items, the new gasoline sales tax drew the most fire when the Greater Washington-Maryland Service Station Association mounted a futile hardball lobbying drive seeking to prevent its passage. Nevertheless, the City Council enacted the legislation by a vote of 11 to 2.

Victor Rasheed, executive director of the association, predicted that the need for the city's 240 service station operators to collect and remit the new tax each month will cause severe problems, especially for smaller dealers. Many, he said, may overextend themselves and come up short of funds at taxpaying time.

The gasoline sales tax will be levied at the rate of 6 percent on top of the pump price, which already includes a District of Columbia gasoline tax of 10 cents a gallon and a federal tax of 4 cents a gallon.

The present gasoline tax is included by the oil companies in the wholesale price of the fuel they deliver to station operators. The oil companies collect the full price from the operators and remit the tax money to the D.C. and federal governments.

Under the new law, the individual stations operators must collect and safeguard the tax money and file reports each month as they remit it to the city.

Carolyn L. Smith, director of the D.C. Finance and Revenue Department, disputed Rasheed's prediction that the new tax will create problems. Service station operators already must file sales tax reports on their receipts from motor oil, accessories and other items, she said, and it should be a relatively simple matter to include gasoline.

Calculating the tax to be collected from individual motorists will be simple, Smith said. The dials on pumps will be adjusted so the amount of gasoline that now sells for $1 will be priced at $1.06. With that adjustment, pumps automatically will show the price for larger amounts.

Smith said most retailers will have no problem collecting and remitting the increased general sales tax, which will go from 5 percent to 6 percent. New tax tables are being distributed to reflect that change.

For the first time under the new law, an 8 percent tax will be collected on candy, chewing gum and soft drinks sold in stores and other retail establishments, which are now exempt from any taxation. These same items are taxed -- and for now will continue to be taxed -- at 2 percent when sold in coin-operated vending machines.

Ernest Moore, public affairs manager for Safeway, the region's second-ranking food chain in sales volume, said the 8 percent rate will create problems for checkers and delays for customers. These items must be sorted largely by hand from food, which is untaxed, and most other merchandise, taxed at 6 percent.

Barry Scher, spokesman for Giant, the largest regional chain, said his firm's computerized scanners will be programmed to levy the different levels of tax.

Cha Ku, the Korean-born owner of a candy and magazine stand in the lobby of the Investment Building, 15th and K streets NW, said the 8 percent would amount to 1 1/2 cents on a candy bar that now retails at 25 cents. "I will charge customers a peny more," he said. "That's too bad, my profit cut."

The tax package also includes a rise in the sales tax collected on hotel and motel rooms from 8 percent to 10 percent. There will be no change in a separately charged hotel occupancy tax of 80 cents a night for each room.

Also going into effect Aug. 1 will be the doubling of the current 1 percent tax on the value of real estate transactions.

The existing tax is paid by the buyer. It is contemplated that the new tax will be paid by the seller. Marguerite Stokes, the city's acting recorder of deeds, who will administer it said the actual payment can be negotiated between the buyer and the seller, with both of them jointly liable for any failure to pay.

Barry's tax package also includes higher tax rates based on the value of business and manufacturing equipment and contemplates a higher tax rate for commercial real estate but not for homes and apartments. These actual tax rates will be set later.