While the national average retail price for a gallon of gasoline is on the decline, regional markets are showing considerable volatility, motor club AAA said.

AAA reports a national average retail price for a gallon of regular unleaded gasoline at $2.70 per gallon, about five cents below this time last week. The price for Tuesday reflects 13 straight days for a decline in retail gasoline prices.

"Most drivers are paying the lowest July prices since 2009 and are saving 82 cents per gallon at the pump compared to a year ago," the motor club said in a weekly market report.

Gasoline prices are in part a reflection of crude oil prices, which are 20 percent lower, or about $12, lower than the start of the month for West Texas Intermediate, the U.S. benchmark. AAA attributed the decline in crude oil prices to an oversupplied market in an economy that was performing below expectations.

As a whole, national prices are moving steadily lower along with crude oil prices. Regionally, however, the situation is considerably volatile, AAA reported.

"Pump prices have moved dramatically higher in California this month due unexpected drawdowns in supplies, shortages in the blending components used to make the state's mandated boutique fuel and recent reports of several refineries dealing with production issues," it said. "On the other end of the spectrum, production at refineries in the Midwest is reported to have reached a 2015 high and prices have subsequently moved noticeably lower in much of the region."

At $3.84 per gallon on average, California gas prices are up about 40 cents from one month ago. At $2.43 per gallon on average, Ohio gas prices are down about 40 cents from one month ago.

The U.S. Energy Information Administration said it expected retail prices to average $2.44 for full-year 2015 and $2.55 for next year.