San Diego ? Drivers across the nation are digging deeper into their wallets to cover rising gasoline prices, which have leapt an average of 23 cents per gallon in the last month  the most dramatic change in more than a decade.

“It hits the pocket book,” 36-year-old Cindy Trujillo said as she pumped gas Thursday in Denver, adding she recently switched from a pickup truck to a fuel-friendly Geo Metro.

The jump to Thursday’s national average of $1.35 for unleaded, according to a AAA survey, is fueled by a combination of factors. Analysts say a recent decision by OPEC and other oil producers to hold down production, and the traditional spring rise in demand as driving time increases with the warming weather have shaped prices.

The four-week leap is the sharpest seen by the Energy Information Administration, the statistical branch of the Department of Energy, since it began keeping records in 1990.

Part of the reason is that gas prices fell to bargain levels  below $1 a gallon in some areas  in the wake of the Sept. 11 terror attacks, which hampered travel and slowed the economy.

“Now that the economy has started to recover, and we’re starting to head into the summer driving season, the industry is really having to come from behind a little bit,” AAA spokesman Geoff Sundstrom said in Orlando, Fla.

Prices still are below the average of March 2001, when it was $1.43 due largely to the then-strong economy.

“It went down so low, we had a bonus there for a while,” said Vesper Gibbs Barnes, a Boston attorney who dropped her car off at a Mobile station. “I guess I’ll keep driving everywhere. I have to deal with it.”

Crude oil prices have risen to about $25 a gallon since December, when OPEC decided the $20 a barrel they were earning then was too low, said Douglas MacIntyre, senior oil market analyst with the Energy Information Administration.

Every $1 increase in the price of a barrel of crude oil translates into a per-gallon hike of about 2.5 cents, he said. Based on current trends, motorists should expect to see per-gallon prices rise another 5 cents to 15 cents during the next several weeks, he said.

Californians, who shoulder the added costs of reformulated gasoline mandated by pollution restrictions, are facing the highest prices in the continental United States. Bay Area motorists have seen average prices rise to $1.68 from $1.42 a month ago.

How far prices will climb exactly is uncertain, said Carol Thorp, spokeswoman for the Auto Club of Southern California. Perhaps Americans who canceled travel plans last year due to high gas prices or Sept. 11 will feel the urge to hit the highways this year, she noted.

“This summer is a question mark at the moment,” Thorp said. “Anyone who tells you they can predict that is not correct.”