GASOLINE PRICES State-by-state gasoline averages from AAA  GAS SAVING TIPS Tire pressure. Driving a car with underinflated tires is like running laps with 5-pound weights strapped to each leg. Underinflated tires can sap 4% to 10% out of a car's potential gas mileage, says John Frala, associate professor of advanced transportation at Rio Hondo College in Whittier, Calif. The correct tire pressure, which might be different for front and rear tires, is found in a car's owner's manual. Sometimes it's pasted inside the glove box door or attached to a door frame. Check the pressure when the tires are cold. When replacing tires, try to buy a new set with the least amount of rolling resistance, Frala says. More tips

The national average price of a gallon of gas hit $4.005 Sunday, up from $3.988 Saturday, and $3.105 a year ago, according to the American Automobile Association. But that's just the average. Drivers in California are paying an average of $4.44 a gallon, AAA says. In Visalia, Calif., a gallon of regular averaged $4.50. Missouri had the lowest average price: $3.80.

A gallon of diesel averaged $4.76, vs. $2.90 a year ago. Premium gas averaged $4.80, vs. $3.61 last year.

People are changing driving habits to squeeze every mile. "I'm getting better mileage because I've slowed down," says Joseph Micalizzi of Staten Island, N.Y. By driving at 45 instead of 60, Micalizzi is getting another 50 to 60 miles between fill-ups.

Some are cutting back in other areas, too. Russell Whittaker of New Bern, N.C., has cut premium cable channels, canceled magazine subscriptions and set the air conditioner at 80 to compensate for rising fuel prices. "The price of gas in and of itself isn't an issue, but how it affects the price of everything else is a problem," Whittaker said in an e-mail on Sunday.

The rise in gas prices follows an unexpected $10.75 jump in the price of oil Friday, to a record close of $138.54 a barrel. It had spiked up as high as $139.12.

Investment bank Morgan Stanley predicted oil prices would hit $150 by July.

Helping to send oil soaring: Israel's transportation minister on Friday said an attack on Iran was "unavoidable." Iran is the world's second-largest producer after Saudi Arabia. It produces 4 million barrels of oil a day.

And the U.S. dollar fell. A euro cost $1.5768 Friday, vs. $1.5593 on Thursday. Oil is priced in dollars; when the dollar falls in value, oil prices rise.

Normally, it takes awhile for oil price increases to affect gas prices. "We are cautioning gas station owners around the country not to overreact to one day of trading," AAA spokeswoman Yolanda Cade says.

For roughly two decades, prices have tapered in late May, often enough to provide a summer respite.

But even if they fall a bit, it's unlikely to last. "I still worry about late July and August when there will be plenty of hurricane hype," Tom Kloza, veteran analyst at consultant Oil Price Information Service, said in an e-mail.

Hurricane forecasts send prices up because energy operations in the Gulf of Mexico could be hit. After Katrina in 2005, prices rose to about $3.06, a record.

Contributing: James R. Healey

Enlarge By Darren Hauck, Getty Images Motorists fill up at a gas station in Wisconsin. The national average price of a gas hit $4.005 Sunday.