Hawai'i gasoline prices could register their biggest decline in three weeks because of an expected decline in the state's wholesale gas cap.

Advertiser calculations show the cap will be cut by about 9 cents next week reflecting declines in wholesale prices on the Mainland.

The state Public Utilities Commission is scheduled to announce next week's cap this morning. Retail prices at O'ahu's lowest-priced stations such as Costco could dip below $2.50 a gallon.

A cut of 9 cents would mark the biggest decrease since Oct. 24 and be the sixth consecutive week prices under the cap have fallen. The estimate is similar to an 8-cent decline predicted by the state House of Representatives. The law that went into effect Sept. 1 took aim at Hawai'i's gasoline prices which historically have been the highest in the nation.

Kailua resident Mary Lerps has consolidated her driving trips as well as started riding her bicycle more because of the high prices. She returned over the weekend from Coeur d'Alene, Idaho, where a gallon of regular was $2.19.

"I went, 'Oh, what can we fill up?' " Lerps said. "I even took a picture of it. The people at the gas station thought I was crazy."

Hawai'i's motorists paid an average of $2.77 a gallon for regular Monday, according to the American Automobile Association. That was 14.4 percent higher than a year earlier.

That compared to the national average of $2.292 a gallon, or 16.3 percent more than a year earlier, AAA said.

Hawai'i's diesel prices continue to lead the nation at $3.428, or about 67 cents more than the national average. Local trucking, fishing and bus companies have said the prices were causing problems.

"It doesn't kill us but definitely we feel it," said Bo Midro, administrator for Pineridge Farms Inc., a trucking company on O'ahu.

He said the company, with a fleet of 80 trucks along with construction site machinery, uses about 1,500 gallons of diesel a day. The fuel bill now averages about $2,000 a week more than before Hurricane Katrina, he said.

The prices are "pretty quick to increase but they're slow to decrease," Midro said.

Diesel prices aren't regulated under the state's gas cap law, which was enacted to tie Hawai'i prices with those on the Mainland. Proponents decried local prices that went up but rarely fell when prices declined in other states. The cap is computed using wholesale prices over a five-day period in New York, the Gulf Coast and Los Angeles.

Hawai'i's gas cap regulates only wholesale prices. Gas stations are free to set their own prices, though they generally move up or down following the cap's direction.

The Advertiser's calculation of next week's gas cap is based on price data from Bloomberg News while the state uses figures supplied by the Oil Price Information Service. The different sources could result in a variation of a few cents.

Reach Greg Wiles at gwiles@honoluluadvertiser.com.