(CNN) -- Average U.S. gasoline prices have dropped more than a nickel over the past two weeks, marking their first dip since December, according to a nationwide survey published Sunday.
A gallon of regular now costs $3.91 on average, the Lundberg Survey found. That's down more than 5 cents from two weeks earlier, when the average was just a few cents below the $4 mark.
"We can thank crude oil for allowing gasoline to do what it has been wanting to do for weeks, which is drop," said publisher Trilby Lundberg.
Gasoline supplies are plentiful, but crude oil prices had been "propping up" gasoline costs, she said. As crude oil prices have stabilized, gasoline prices have fallen.
"We may see another five to 10 cents decline in the next few weeks" if crude oil doesn't rise, she said.
Even looking ahead to later in May, as the country enters peak driving season, prices might not jump if crude oil stays stable because gasoline supplies are flush, Lundberg said.
The average price nationally one year ago was $3.88 -- just three cents under the current average, Lundberg said.
The Lundberg Survey tabulates prices every two weeks at thousands of gas stations nationwide.
Over the past two weeks, the average price in Chicago has fallen nearly 19 cents, to $4.26 -- but that's still the highest of any city in the latest survey, taken Friday.
The city with the lowest average gas price was Tulsa, Oklahoma, at $3.52.
Here are prices in some other cities:
Memphis, Tennessee: $3.72
Portland, Oregon: $4.02
San Francisco: $4.21