Gas prices dropped slightly in the past two weeks as refineries finished projects that temporarily caused a dip in production, the Lundberg Survey (search) said Sunday.

The weighted national average price for all three grades of gasoline fell a little less than a penny between July 9 and Friday to $1.95 per gallon, said Trilby Lundberg (search), who publishes the semimonthly Lundberg Survey. The survey polls nearly 8,000 gas stations across the United States.

That represents a drop of 15.4 cents per gallon since May 21, when the average price was slightly above $2.10 a gallon.

The biggest seller in the two-week period was self-serve regular, which averaged $1.92 per gallon. Mid-grade gasoline averaged $2.02 per gallon, while premium averaged $2.11 per gallon.

Lundberg said refineries had finished several projects designed to maximize output for summer travel, leading to the drop in prices. She said an increase in crude oil prices prevented prices from falling even further.

The lowest average price for a gallon of self-serve regular was $1.74, in Tulsa, Okla. The highest average price for self-serve regular was in Honolulu, where consumers are paying $2.25 per gallon.