The average price of a gallon of gasoline in the Washington area has fallen 10.4 cents in the last two months, the sharpest drop ever measured in surveys by the American Automobile Association's Potomac Division.

Gasoline now sells for an average of $1.304 a gallon, counting all grades of fuel and both full-service and self-service pumps, compared to the Feb. 1 average of $1.408 a gallon, according to AAA representative Lee Cooke. Those averages reflect prices this week at 80 major-brand stations scattered randomly through the metropolitan area, Cooke said.

Prices for gasoline vary widely, from 99.9 cents a gallon for regular leaded to $1.689 a gallon for premium unleaded.

Several area stations have begun offering 99-cent gasoline during the last week, but not apparently on a permanent basis. The Allentown Texaco at 7713 Allentown Rd., Prince George's County, sold regular leaded gasoline at 99.9 cents a gallon over the weekend. But dealer Gary Fleming raised the price to $1.049 a gallon on Monday because he said he was losing too much money.

Yesterday, the Hess station at Rte. 1 and Backlick Road in Fairfax County sold regular leaded gasoline for 99.9 cents a gallon as a protest by dealer Arthur Swartz against oil company wholesale pricing procedures. But that was a one-day special. Today, he said, the price will be back to $1.109 a gallon.

In the District of Columbia, Capitol Hill Gulf, 200 Massachusetts Ave. NE, now sells regular leaded gasoline for 99.9 cents a gallon. But operator Ben Simpson said he may mark the price back up in a few days if he continues to lose money. In the meantime, to offset the losses, he has marked up regular and premium unleaded gasoline by 10 cents a gallon to $1.259 and $1.369 a gallon, respectively.

Only one station indicated it is keeping its 99.9-cent-a-gallon gasoline indefinitely, and that station is in Rockville, relatively far for many area residents to reach. It is the Geddes Texaco, 806 Rockville Pike.

One result of the increased competition among dealers is an increase in the number of complaints from motorists about stations that post one gasoline price on a sign visible from the street but actually charge a higher pump price.

The problem typically occurs at stations that dispense gasoline by the liter from the pump but post large price signs based on gallonage, according to area officials.

On March 26, a Maryland dealer was convicted and fined $200 for misrepresenting the price of his gasoline, according to Lacy DeGrange, assistant chief of the state's weights and measures section. DeGrange said the Cee's Shell in Seat Pleasant had set its pump price at 33.9 cents a liter. That translates to a price of $1.283 a gallon, DeGrange said. But the station had posted a large sign stating that its price was $1.249 a gallon, a difference of 3.4 cents a gallon.

DeGrange said dealers sometimes make small mistakes when they try to work out the price per gallon when they are selling by the liter. The mistake occurs when numbers are rounded off incorrectly, he said.

Motorists who are uncertain if they are paying the posted price can compute the correct price for themselves, DeGrange said. Since there are 3.785 liters in one gallon, multiply the pump liter price by 3.785. The answer should be no more than the posted price per gallon.

The cost of gasoline is lower now than it has been since Dec. 8, 1980, when the average price was $1.284 a gallon, according to the AAA.

The average price reached a peak on April 3, 1981, when it was $1.454 a gallon, and has been dropping steadily since then.