The average price for a gallon of gasoline in the United States rose 3.74 cents over the past two weeks, the smallest increase since January, according to the nationwide Lundberg Survey.

The national average of a gallon of regular gasoline rose to $3.9671 on April 6, according to the survey of about 2,500 gasoline retailers.

That was smaller than the 11.49-cent increase in the prior survey for the two weeks ended March 23.

"If crude oil will stay where it is, then gasoline prices will peak very, very quickly, if they have not done so already," survey editor Trilby Lundberg told Reuters on Sunday.

Gas prices already are falling in some cities, such as Chicago and Los Angeles, Lundberg said.

"They are dropping in many markets of the West and the Midwest," Lundberg said.

Lundberg also said significant unused capacity exists at U.S. refineries in case demand spikes.

"We are heading into the stronger demand season, but with all that unused capacity there should be a continued abundance of gasoline supply," Lundberg said.

Chicago tallied the highest average price among cities covered by the survey, at $4.45 per gallon, although that marked a 11-cent drop from the prior survey. Tulsa, Oklahoma was the lowest, at $3.66.