New York - Retail gasoline prices are poised to cap the biggest July decline in six years as U.S. refineries process the most oil on record, AAA says.

The cheapest gasoline Thursday was in South Carolina, where drivers paid an average of $3.25. The highest price was in the continental United States were in California at $3.98.

The decline is helping consumers during the peak-demand summer months, when Americans take the most vacations, and comes as data indicate the U.S. economy is improving. Gross domestic product rose at a 4 percent annualized rate in the second quarter after shrinking 2.1 percent from January through March, Commerce Department figures showed Wednesday.

Regular gasoline has dropped 15.5 cents, or 4.2 percent, to $3.517 a gallon this month, according to Heathrow, Florida-based AAA, the nation's largest motoring club. Prices have averaged $3.60 a gallon, down from $3.67 in June, after rising more than 4 percent in the three previous Julys, AAA data show.

"It's very unusual to see gas prices fall at this time of year," Michael Green, a spokesman for the Heathrow, Fla.-based automobile club AAA, said by telephone from Washington. "This is nearly the reverse of what we saw in the previous three years when prices increased considerably in July."

U.S. refineries, which operated at more than 93 percent of capacity for the past three weeks, processed 16.5 million barrels a day of crude in the four weeks ended July 25, Energy Information Administration figures show. It's the highest in data going back to May 2005.

Gasoline inventories have increased eight of the past nine weeks. They reached 218.2 million in the week ended July 25, a four-month high.

Crude futures have dropped in July as the surge in violence in Iraq and Ukraine had little impact on the flow of oil. West Texas Intermediate climbed to a nine-month high in June after militants from a breakaway al-Qaida group known as Islamic State captured the city of Mosul.