Hawai'i's wholesale gasoline cap is poised to fall for a sixth consecutive week after prices on the Mainland dropped to their lowest levels since the beginning of summer.

Hawai'i drivers could see a decline of about 8 cents a gallon next week, according to preliminary Advertiser calculations. The price for a gallon of regular at O'ahu's least expensive stations, such as Costco, could dip below $2.50 a gallon, the lowest since mid-August.

Wholesale prices under the state's gas cap law have fallen about 80 cents in the past five weeks, the most recent cut coming yesterday when the cap declined by about 3 cents. The law took effect Sept. 1 and is an attempt to get Hawai'i gasoline prices tracking those on the Mainland.

Tomorrow the state will announce the official cap for next week.

Even with six straight weeks of declining prices, gasoline still costs too much, said Marty Martinez of Kailua, Kona, on the Big Island.

"They're using a gas nozzle instead of a gun," said Martinez, 72, who combines his trash-dump runs with trips to buy gas at Costco. "You've got two choices — either you take it or leave it."

The caps haven't changed the fact that Hawai'i has the nation's highest gasoline prices.

According to the American Automobile Association's Daily Fuel Gauge report, Hawai'i motorists paid an average of $2.794 for a gallon of regular on Sunday night compared with the nationwide average of $2.293.

Hawai'i's gasoline cap is the only one in the nation and is based on wholesale prices in New York, the Gulf Coast and Los Angeles.

The law was enacted because local prices often increased but rarely dipped when prices on the Mainland declined, proponents say.

The newest drop comes as prices in New York and the Gulf Coast fell to their lowest levels in five months. Los Angeles spot market prices reached their lowest since the first quarter.

Diesel fuel continued to hover near a record high in Hawai'i, with the average price of $3.47. That was 69.4 cents a gallon more than the national average, or among the biggest differences between the state's and U.S. averages since the start of 2004, according to data from the Hawai'i Department of Business, Economic Development and Tourism.

"We get hit with high gas prices, and then hit again with higher costs of goods and services provided by diesel powered delivery/construction vehicles," diesel truck owner Bill McConnell wrote in an e-mail.

Hawai'i's gasoline prices may not be high enough to alter people's driving habits, state Department of Taxation figures show. More than 42.4 million gallons of gasoline were sold in the state in September.

That was about 5 million gallons more than a year earlier but down from the 50.8 million sold in August, data computed from gasoline taxes show.

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 gas cap is based on five days of wholesale price data from three Mainland markets. The Advertiser's preliminary calculation of next week's gas cap is based on the first four days of the five-day period. The Advertiser uses 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.