The national average gasoline price fell almost 5 percent to a two-year low in the past month, according to the American Automobile Association. It was the biggest monthly decline since 1991.

The price of self-serve regular unleaded gasoline fell to $1.159 a gallon, down 5.7 cents, or 4.7 percent, from December. The price is 12.9 cents, or 10 percent, less than it was a year ago, according to AAA’s monthly Fuel Gauge Survey, and is the lowest since Jan. 20, 1996.

“It’s the biggest drop since the end of the Persian Gulf War,” said Mike Morrissey, AAA spokesman.

The average price of self-serve mid-grade unleaded is $1.241 per gallon, 6.3 cents less than last month and 13 cents less than last year. Self-serve premium is $1.307, down 5.1 cents from a month ago and 13.2 cents less than last year.