The average gasoline price nationwide for all grades tumbled 6 cents in two weeks, continuing a slide in pump prices that began last month, an industry analyst said Sunday.

The average retail price for all three grades dropped 6.37 cents to $2.18 per gallon between May 6 and Friday, said Trilby Lundberg (search), who publishes the semimonthly Lundberg Survey (search) of 7,000 gas stations around the country.

The most popular grade, self-serve regular, was priced at $2.15 a gallon, while customers paid $2.25 for midgrade. Premium averaged $2.35 a gallon for the period.

Average gasoline prices (search) spiked 49 cents per gallon between Jan. 1 and April 8, when prices began to drop as a result of several factors including an increased supply of both crude oil and gasoline and the completion of some maintenance projects that had reduced capacity at the nation's refineries.

Oil prices began skyrocketing in March, hitting a peak of $57.27 a barrel at the beginning of April. Prices have since receded and now hover above $47 a barrel.

The highest average gas price in the nation for regular unleaded among the stations surveyed was $2.51 a gallon in San Francisco. The lowest was $1.94 in Jackson, Miss.