(CNN) - Gas prices rose more than 8 cents over the past two weeks, hitting a U.S. average of $2.21 per gallon for self-serve regular in what could be a prelude to higher costs for motorists in 2006, a national survey concluded Sunday.

"It's the first increase in three-and-a-half months," said Trilby Lundberg, whose Lundberg Survey compared prices at about 6,000 gas stations on Dec. 15 and Dec. 2.

Prices had tumbled nearly 88 cents per gallon since they peaked at $3.01 on Sept. 9, after Hurricane Katrina damaged many of the refineries on the Gulf Coast. Lundberg said the 8.01-cent increase "comes from a moderate rebound in both crude oil prices and in gasoline demand."

All but 3 percent of the nation's refining capacity is back on line, with just two refineries still undergoing repair, she said.

She predicted further increases in the 2- to 5-cent range over the next few weeks, but said they would pale in comparison to what will happen in 2006.

"After the New Year begins, there could be big-time price pressure on gasoline," Lundberg said.

She predicted "steep rises" of about 57 cents per gallon through July as government-mandated reductions in sulfur content go into effect Jan. 1; a federal requirement that American gasoline include 4 percent ethanol; and the absence of legal liability protection for refiners of MTBE, an additive that boosts octane levels but has been blamed for water pollution.

Salt Lake City drivers paid the least, at $1.98 per gallon of self-serve regular; Honolulu drivers the most, at $2.57, she said.

Here are some other prices:

Atlanta: $2.14

Houston: $2.16

Seattle: $2.17

Los Angeles: $2.21

Hartford, Ct.: $2.27

Chicago: $2.34

During the year, gas prices have risen more than 40 cents per gallon, from $1.79 on Jan. 7, Lundberg said.

She said that more than 80 percent of the price hike was attributable to increases in the cost of crude oil; 10 percent went to fatter profit margins for refiners, and the remaining 10 percent went to state tax collectors and retailers.