In another sign of just how expensive gasoline is getting, the Honolulu Police Department yesterday said it will switch from premium gasoline to mid-grade on Oct. 1 to save the city money.

The switch of all 1,500 of its vehicles to cheaper mid-grade gasoline is expected to save the department about $10,000 a month, said police Capt. Frank Fujii.

"Gas prices are going up for everybody," he said. "Like everybody, we're at the mercy of gas."

Premium gasoline has a higher octane rating than mid-grade or regular. However, there is typically no added performance benefit to premium unless a car's engine requires the added octane to prevent engine knock, according to the Federal Trade Commission.

The average price for a gallon of premium gasoline in Honolulu was $3.863 cents, according to yesterday's AAA Gas Gauge. Mid-grade was $3.792, and regular was $3.586.

Only about 6 percent of cars sold in the United States require premium gasoline, a grade required to meet octane standards of high-performance, high-compression engines, according to AAA travel club. All gasoline grades contain a federally mandated level of detergents aimed at reducing engine buildup.

Honolulu's main police cruiser is the Ford Crown Victoria, which does not require premium or mid-grade gasoline. The California Highway Patrol runs the same cars on regular gas.

Honolulu police decided to go with mid-grade gas instead of regular because some of the high-performance vehicles driven by police as part of the subsidized vehicle program require higher octane, said Darryl Higa, vehicle maintenance superintendent for the department. As of a year ago, the department subsidized about 1,200 private vehicles.

The department does not have the ability to store and distribute more than one grade of gasoline, Higa said.

The switch to mid-grade is on a trial basis, Fujii said.

Fujii said Honolulu police experimented with using lower-octane gasoline 10 years ago. However, the older vehicles used at that time suffered from engine-knock when running the lower-grade gasoline.

Honolulu police spent $2.98 million on gasoline during the 12 months ended June 30, which was up from $2.48 million during the previous fiscal year.

Reach Sean Hao at shao@honoluluadvertiser.com.