Retail prices for gasoline are moving lower thanks in part to the increase in production from North American shale, motor club AAA said.

AAA said in its weekly roundup of retail gasoline prices that consumers in the United States paid the lowest prices on average in September in four years, with a monthly average of $3.39 per gallon.

That's 13 cents less than September 2013 and 44 cents less than the same month two years ago.

Gasoline prices start a seasonal decline in September because the summer travel season is over and refineries start making a winter blend of gasoline, which is less expensive to produce.

Avery Ash, a spokesman for AAA, said gasoline prices should continue their decline, with the national average price for a gallon of regular unleaded expected to drop another 20 cents by the end of November.

"If everything goes smoothly, buying gas for less than $3.00 per gallon should be refreshingly common in many parts of the country this winter," he said in a statement Tuesday.

Though refinery issues play into the price at the pump, AAA said gasoline prices in general have been less expensive than in years past because of the increase in production from shale basins in North America.

"U.S. refineries have taken advantage of increased crude oil supplies to make more gasoline," the motor club said. "In addition, increased domestic production has helped insulate U.S. consumers from conflicts and instability overseas."