(CNN) -- U.S. gas prices showed their smallest increase since early January -- and may be nearing or past their peak if the price of crude oil holds steady, according to a national survey published Sunday.
The average price for a gallon of self-serve gasoline was $3.97 as of April 6, the Lundberg Survey found. The price is up 3.74 cents since the previous survey on March 23 -- a "comparatively small" hike to those seen recently, publisher Trilby Lundberg said.
"The two reasons are the continued flush supply of gasoline and the fact that, in this two-week period, crude oil prices fell," she said. "... Crude oil is the main driver of whatever gasoline prices do. If crude oil stays more or less where it is now, the price hikes will cease. If it goes back up again, we will resume climbing."
Crude oil prices are down $3.56 in the past two weeks, from $106.87 on March 23 to $103.31 on April 5, she said.
Some cities saw sizable hikes in gas prices over the past few weeks, while others saw notable drops, Lundberg said. Chicago, for instance, saw prices fall nearly 11 cents since March 23 -- but still had the nation's highest price, at $4.45 per gallon. Cleveland's gas prices were down more than 11 cents in the past two weeks to $3.79 on April 6.
The lowest U.S. gas prices were seen in Tulsa, Oklahoma, with $3.66 per gallon, she said.
Average per-gallon prices in some other cities:
Birmingham, Alabama: $3.77
Salt Lake City: $3.67
San Diego: $4.26