(CNN) -- Gas prices have fallen over the past two weeks, continuing their slide despite an increase in crude oil prices, according to a survey published Sunday.
The average price of regular gasoline is $3.29 a gallon, the Lundberg Survey found.
That's down 9 cents from two weeks earlier, and down a total of 18 cents over the past six weeks, said publisher Trilby Lundberg.
While crude oil prices rose $3.55 a barrel over the past two weeks, gas prices fell due to "shrinkage in the profit margins of the downstream portion of the oil industry -- that is, refiners and retailers," Lundberg said. "They were unable to pass through the higher oil prices because American motorists' demand for gasoline continues to shrink due to hard economic conditions."
However, it's unlikely prices will continue to fall, Lundberg said., because refiners and retailers "will not be able to sustain losses on and on."
"It will all depend on what crude oil does," she added.
The latest average gas price is 38 cents higher than it was a year ago.
The Lundberg Survey tallies prices at thousands of gas stations nationwide.
The city with the lowest average price in the latest survey was Albuquerque, New Mexico, at $2.84. The highest average was in San Francisco, at $3.67.
Here are average prices in some other cities:
-Charleston, South Carolina - $3.08
-Houston - $3.09
-Atlanta - $3.22
-Denver - $3.23
-Boston - $3.39
-Chicago - $3.48
-Seattle - $3.56
-Tucson, Arizona - $3.04
CNN's Josh Levs contributed to this report.