The average price of a gallon of gasoline at the pump increased more than 2 cents during the past two weeks, the Lundberg Survey said.
The survey added that future prices are impossible to predict due to the Middle East crisis.
Trilby Lundberg said a survey Friday of more than 13,000 retail outlets around the country showed that the average price of a gallon of gasoline, including taxes, was 135.44 cents.
That was an increase of 2.28 cents from a survey Aug. 24.
"That is a very strong price behavior for this time of year, although it sets no precedent," Lundberg said.
"Prices are often pulled down at this time of year due to the end of the summer driving season. The peak consumer demands are in July and August, and the price normally dips due to the drop in consumption."
"This year," she said, "it is impossible to say how much the lower consumption impact will have on retail prices. The higher prices for crude and refined products are still at work in the domestic and international markets due to the Middle East crisis."
In the past month, the average price of a gallon of gasoline in all grades has increased 7.5 cents at American stations.
In the latest survey, the price at self-service pumps for a gallon of unleaded regular gasoline was 124.28 cents.
Premium unleaded was going for 140.99 cents and regular leaded was at 123.74 cents.
At full-service pumps, the price of unleaded regular was 146.91 cents per gallon.
The going rate for premium unleaded was 160.60 cents and for regular leaded was 146.78 cents.