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(CNN) -- Gas prices rose more than 9 cents over the past two weeks, to a national average of $3.20 per gallon of self-serve regular, a survey said Sunday.

The Lundberg Survey, which compared prices tallied March 7 with those tallied February 22 at about 5,000 gas stations nationwide, found the highest prices in San Francisco ($3.58) and the lowest in Cheyenne, Wyo. ($2.99).

Publisher Trilby Lundberg warned motorists not to expect the prices to fall anytime soon.

"We should be ready to absorb even higher prices, and quickly," she said, citing last week's decision by the Organization of Petroleum Exporting Countries (OPEC) not to raise production; growing world demand for oil; the fact that U.S. refiners have held back passing along costs; and the "mileage penalty" associated with government-mandated increases in the use of ethanol in gasoline.

"As the mandate brings more ethanol into the pool, the mileage penalty grows," she said.

Last year, when 4.3 percent of gasoline was ethanol, vehicles got 1.4 percent less miles per gallon than they would have on non-ethanol gasoline, she said. This year, with a mandate of 6.3 percent, the average MPG will be reduced by 2.1 percent, she said.

Daylight Saving Time will bring a growth in demand that will remain in force throughout the summer, and could result in price increases of "at least 20 to 30 cents at the pump, perhaps by mid-April," she said.

Oil refiners' tighter margins can be credited with the fact that prices are not even higher, she said. This year, refiners' margins per gallon are 25 cents less than what they were last year, she said.

Prices in California and other high-priced markets could reach $4 per gallon "in the next few months, if crude oil prices hold," she said.

The recent price surge has affected diesel fuel more than gasoline. Diesel drivers paid an average of $3.80 per gallon, up 21.52 cents from two weeks ago and up $1.02 from a year ago, she said.

Over the past year, the price of gasoline has risen 64 cents, she added.

Lundberg said federal regulations mandating the removal of sulfur from diesel products have increased its refinery costs.

Here are some other cities' prices for gasoline:

-- Saint Louis, Missouri: $2.99

-- Denver, Colorado: $3.00

-- Baton Rouge, Louisiana: $3.07

-- El Paso, Texas: $3.09

-- Baltimore, Maryland: $3.14

-- Detroit, Michigan: $3.21

-- Hartford, Connecticut: $3.27

-- Seattle, Washington: $3.46

-- Long Island, New York: $3.30