NEW YORK — U.S. retail gas prices extended their record run Thursday, adding to the pain consumers feel every time they fill up. Experts predict prices will rise even higher as peak summer driving season approaches.

Meanwhile, crude oil futures fluctuated as investors weighed crude oil's latest records and looked to the dollar for direction.

At the pump, the national average price of a gallon of gas rose 1.4 cents overnight to a record $3.357 a gallon, according to AAA and the Oil Price Information Service. Prices have set a string of records in recent weeks, and are 56 cents higher than a year ago.

Retail diesel, the fuel of trucks and other heavy vehicles, rose overnight to a new national record of $4.045 a gallon.

The Energy Department expects gas prices to average as much as $3.60 a gallon this summer, but believes the national average price could spike as high as $4 a gallon at times.

"Gas hitting $3.60, $3.65 a gallon seems like a done deal," said James Cordier, president of Tampa, Fla., trading firms Liberty Trading Group and

In part, gasoline prices are rising due to a supply crunch that occurs every spring when refiners switch over from making winter grade gasoline to the less polluting fuel they're required to sell during the summer. Summer grade gasoline is more expensive to make. Also, refiners try to sell off all of their winter grade fuel before the switchover, which drops supplies to very low levels.

This year, the spring price spike is being exacerbated by two unusual factors: tight supplies of key gasoline blending components and record oil prices. Analysts say alkylate, an ingredient critical to the manufacture of summer grade gasoline, is in short supply and will push prices higher.

Meanwhile, crude oil, the main ingredient in gasoline, is engaged in its own record run.

On Thursday, light, sweet crude for May delivery fell 49 cents to $110.38 a barrel on the New York Mercantile Exchange, but alternated between gains and losses. Crude prices rose to a new trading record of $112.21 on Wednesday after the Energy Department said supplies fell unexpectedly last week.

The dollar stabilized against the euro on Thursday, giving some investors an excuse to take some profits from the previous session's rally, analysts said. A weaker dollar had supported Wednesday's rally.

Investors view commodities such as oil as an effective hedge against inflation and a falling greenback. Also, oil is cheaper to investors overseas when the dollar is falling. Many analysts place most of the blame for oil's run above $100 in recent months on the steadily falling dollar.

However, analysts said the market on Thursday was torn between the profit taking and investors plowing money into crude futures on expectations that the dollar will weaken later in the year.

The Federal Reserve is expected to cut interest rates at least twice more this year — moves that are expected to further weaken the dollar — and many analysts believe speculative investors will continue buying crude, boosting prices further.

"We could now see a lot of 'system money' join the upside breakout, propelling prices even higher," said Edward Meir, an analyst at MF Global UK Ltd., in a research note, referring to hedge fund investors.

In other Nymex trading Thursday, May gasoline futures fell 1.43 cent to $2.7599 a gallon, while May heating oil futures rose 3.10 cents to $3.2655 a gallon. Heating oil futures are trading at record levels due to falling supplies and strong demand overseas.

Nymex natural gas for May delivery rose 20.4 cents to $10.26 per 1,000 cubic feet. The Energy Department said natural gas supplies fell by 14 billion cubic feet last week, in line with analyst expectations.

In London, May Brent crude fell 26 cents to $108.21 on the ICE Futures Exchange.