(CNN) -- After nearing the all-time record, gas prices in the United States are sliding downward, with another drop possible in the next few days, according to a survey published Sunday.
The Lundberg Survey found the average price of a gallon of self-serve regular gasoline nationwide was $3.91 as of Friday, publisher Trilby Lundberg said. The price represents a drop of just over 9 cents from May 6, when prices hit the $4 mark.
The all-time high for U.S. gas prices was $4.11 on July 11, 2008, she said.
The drop "comes from the crude oil price slump earlier this month," Lundberg said, "and it looks like there's perhaps another dime drop at the pump in the next few days."
However, whether the drop is more or less than a dime depends on several factors, including what crude oil prices do next and whether flooding in the Southeast, which has caused transport delays, does not worsen, she said.
Asked whether crude oil prices may rise again, Lundberg said, "It certainly is possible at any time." However, the recent high prices have contributed to a dampening of world demand, she said, and demand at the pump in the United States is down "with a double whammy" -- unemployment and underemployment, as well as the Southeast flooding.
The Friday price is about $1.08 higher than the price a year ago -- $2.83 on May 21, 2010.
The city with the highest average price in the latest survey was Chicago, at $4.38 per gallon, Lundberg said. The lowest was Jackson, Mississippi, at $3.60. Different tax levels in different states are a major factor in bringing about diverse prices.
Here are some other average prices in a panel of large cities:
-- Salt Lake City $3.67
-- St. Louis $3.74
-- Houston $3.79
-- Atlanta $3.91
-- Boston $3.98
-- San Francisco Bay area $4.17