The national average price of a gallon of unleaded gasoline fell to $3.457 per gallon for the week ending Sept. 8, according to federal data.

The average price has declined in nine of the past 10 weeks and reached a level  below $3.50 typically seen in winter months, reports the U.S. Energy Information Administration. Gasoline now costs 13 cents less than it did a year ago.

For the week,the price of gasoline fell in five of the nine regions tracked by the agency. The steepest decline came on the West Coast where it fell 2.3 cents to $3.774 per gallon. The average price of gasoline is now at least $3.50 in 20 states and the District of Columbia, according to AAA's Daily Fuel Gauge Report.

Meanwhile, the national average price of diesel remained flat for the week at $3.814 per gallon. Diesel now costs 16.7 cents less than it did a year ago.