Gas prices rise 9 cents in just two weeks, a survey has found.

The average price of self-serve regular gasoline on Friday was $3.19 a gallon, mid-grade was $3.31 and premium was $3.42, according to the Lundberg Survey of 7,000 stations nationwide released Sunday.

Of the cities surveyed, the cheapest price was in Cheyenne, Wyo., where a gallon of regular cost $2.95, on average. The highest was in San Francisco at $3.58.

Compared to a year ago, national gas prices are up 64 cents.