The average price of a gallon of unleaded gasoline has fallen to a level not seen in almost four years, as it now costs an average of $3.12 per gallon, according to federal data.

The average price fell 8.7 cents for the week of Oct. 20 to reach a level not seen since it reached $3.11 on Jan. 31, 2011, according to the U.S. Energy Information Administration. The price is now 24 cents cheaper than a year ago.

The price of gasoline fell in all nine regions tracked by the agency, and saw a decline of at least 10 cents in four of the regions. The sharpest decline came in the West Coast without California where the price fell 11.7 cents to $3.311. The West Coast, Lower Atlantic, and East Coast also saw at least a 10-cent fall.

The average price of gasoline now costs at least $3.50 in only three states, including Hawaii ($4.084), Alaska ($3.801), and California ($3.502), according to the AAA's Daily Fuel Gauge Report.

Meanwhile, the average price of diesel fell 4.2 cents to $3.656 per gallon for the week. Diesel now costs 23 cents less than it did a year ago.