Photo by Vince Taroc.

Photo by Vince Taroc.

The national average price of unleaded gasoline declined 3 cents to $2.39 per gallon to reach a level that's 9 cents higher than a month ago and 18 cents higher than a year ago, according to AAA.

The higher gas prices compared to last year are the result of an OPEC agreement that constrained supply. Substantial increases haven't arrived at the pump due to increased gasoline inventories and low demand across the country, according to AAA.

The least expensive states include South Carolina ($2.09), Oklahoma ($2.10), Mississippi ($2.15), Tennessee ($2.15), Arkansas ($2.15), Alabama ($2.16), Missouri ($2.18), Louisiana ($2.18), Virginia ($2.20) and Kansas ($2.22).

The states with the largest weekly decreases include Indiana (13 cents), Ohio (10 cents), Michigan (10 cents), Kentucky (7 cents), Illinois (6 cents), Oklahoma (5 cents), Delaware (4 cents), Florida (4 cents), Kansas (4 cents) and Wisconsin (4 cents).

Meanwhile, the average price of diesel fell 1.2 cents to $2.583. Diesel is now 31.7 cents higher than a year ago, according to the U.S. Energy Information Administration.