The average price of gasoline in the U.S. is up four cents over the past three weeks to $2.21 a gallon for regular grade.

Industry analyst Trilby Lundberg said Sunday that higher crude oil costs caused refiners and retailers to bump up their prices.

The average price is 23 cents lower than a year ago.

The Lundberg Survey found the average price of midgrade unleaded is $2.49 a gallon, and the average price of premium is $2.71 a gallon.

The highest average price for regular gas in the contiguous U.S. is $2.71 a gallon in San Francisco. The lowest is $1.91 in Jackson, Mississippi.

The U.S. average diesel price is $2.39 per gallon, up about four cents from three weeks ago.