The average price for a gallon of regular gasoline in the United States fell 15.9 cents to $3.624 in the past three weeks due to a drop in crude oil, which took a hit from fears over Europe's economy and a stronger dollar, the Lundberg Survey showed. The national average for regular gasoline as of June 8 was down more than 11.62 cents from the year-earlier level of $3.74 a gallon, according to the survey of some 2,500 gasoline stations in the continental United States.