The US national average price for a gallon of regular unleaded gasoline was $3.25 Monday down 3 cents from the same time last week, the AAA reported. The price is the lowest since Dec. 28, 2012, the automobile association said. It said approximately 13 percent of the gasoline stations in the United States are charging less than $3 per gallon. Missouri is the only state with an average price below the $3 per gallon mark, the AAA said. Avery Ash, an AAA spokesman, said in Friday's report for October gasoline prices the national average price should continue its decline. "There are fewer frowns on drivers' faces as they leave gas stations given recent price declines," he said. "Abundant supplies, declining demand, falling oil costs and the switch to winter-blend gasoline have helped push gas prices down in every single state." The AAA said demand declines after the Labor Day holiday and refiners in September can start making a winter-blend of gasoline, which is cheaper to make than the summer blend because of fewer environmental restrictions. The association said it expects the national average price for a gallon of regular unleaded to reach $3.10 before the end of the year.