NEW YORK – The average cost of regular gas hit $4 a gallon nationally for the first time Sunday, and prices are expected to keep climbing, especially after oil prices neared $140 a barrel in a record-shattering rally Friday.

While Americans who have to drive will feel the biggest squeeze, the increased prices also mean higher costs for consumers and businesses.

“I don’t think we’ve felt quite the full impact of $138 or $139 a barrel oil,” said Jason Toews, co-founder of fuel price research site GasBuddy.com.

Gas prices have risen by about 20 cents in the past three weeks, according to a report by the Lundberg Survey released Sunday. Nationally, they rose overnight from $3.988 to $4.005 a gallon, according to AAA and the Oil Price Information Service.

Drivers in Washington state and many parts of the country have already been paying well above that price for some time. The average price Sunday for regular in Washington state was $4.218, up from $3.767 a month ago, and up from $3.292 a year ago.

In California, a gallon averages $4.436 a gallon, the priciest in the country. Missourians are paying the least at the pump: $3.802 a gallon.

High fuel prices weren’t stopping Todd Swanson of Tacoma from taking a trip Sunday to Eastern Oregon, though they did slow him a bit.

Because registers at the 7-Eleven at Tacoma’s South 19th and Trafton streets allowed for a maximum purchase of $100 at a time, Swanson had to break up his efforts to top off his recreational vehicle. Two trips inside and one debit-card swipe allowed him to put more than $250 of fuel into his 80-gallon tank.

“I save by taking the bus to SeaTac to work every day,” Swanson said. But if gas prices get much higher, his recreational driving will be relegated to “local trips.”

At the next pump over, Tacoma’s Jason Scott was grumbling about spending $3 just to mow his lawn as he pumped less than a gallon of gas into a small container.

“At least it’s not as bad as this monster,” he said, nodding to his older Jeep. “It only gets 9 miles per gallon.”

Truckers and others with diesel engines under the hood have it even worse. A gallon of diesel now sells for $4.762, up nearly a penny overnight, according to AAA and OPIS. Prices hit a record of $4.79 at the end of May. (Sunday’s average price in Washington state was $4.89.)

CRUDE OIL STILL SURGING

Skyrocketing oil prices, which are trading at more than double their level last year, are largely to blame for the surge. Crude prices shot up more than 13 percent late last week, to $138.54, on the New York Mercantile Exchange.

Yolanda Cade, managing director of public relations at AAA, said gas prices are likely to rise, though the organization is holding off on making any predictions.

“We’ve cautioned gasoline station owners against not recklessly increasing retail prices just because of one big jump in the crude market,” she said Sunday. “One day of trading doesn’t constitute a market trend.”

A number of factors are behind oil’s ascent.

Soaring demand in Asia and elsewhere is ensuring global supplies remain tight even as Americans cut back. Recent figures from the U.S. Energy Department’s Energy Information Administration showed that U.S. gasoline demand fell 1.4 percent over the past four weeks.

A tumbling dollar is also contributing to the increase. Many traders buy commodities like oil as a hedge against inflation when the dollar is falling, and a weaker dollar makes oil cheaper for investors dealing in other currencies.

The rapid increase has also enticed speculators frustrated by low returns elsewhere and looking to make a quick profit.

IT’S BEEN WORSE

Economists say that despite widespread concern about gas prices, the nationwide impact of the oil crisis has so far been gentler than during the oil crises of the 1970s and 1980s, when shortages caused long lines at the pump, set off inflation and drove the economy into recession.

Nationwide, Americans now spending about 4 percent of their take-home income on gasoline. The highest point since World War II was 4.5 percent in 1981, according to Brian Bethune, an economist at Global Insight, a forecasting firm. At its lowest point, in 1998, that share dropped to 1.9 percent.

For many drivers, the higher gas prices mean getting creative.

Back in Tacoma, Scott, a Realtor and entrepreneur, has dreamed up a plan to help people save on gas and put some money in his pocket. At his Web site, GasForFREE.com, visitors who meet general requirements can sign up to have their cars “wrapped” in body-tight advertisements in exchange for as much as $300 worth of gas per month.

About 150 people have agreed to participate so far, Scott said.

“It’s not going to eliminate gas problems,” he said of his plan, “but it will take a chunk out of them.”

News Tribune staff writer Bill Hutchens and The New York Times contributed to this report.