TRENTON, NJ -- Most of the nearly 1,700 bi-fuel cars in the New Jersey state motorpool that can also run on natural gas are often being filled with gasoline, and more than half have never been filled with the cleaner-burning natural gas, according to a state audit reported by Newsday. The state audit also said another 250 of the vehicles had seen less than 10 gallons of compressed natural gas fuel over their lifetimes. A New Jersey state treasury spokesman blamed a lack of infrastructure as a reason for reliance on gasoline, the Newsday report said. Over the past seven years, New Jersey has spent $9 million to buy the vehicles, which cost about $5,000 more than regular gasoline-fueled cars. As of 2002, the state had purchased 1,692 of the vehicles. Most of the cars were bought under the Energy Policy Act of 1992 requiring states to make 10 percent of new-vehicle purchases units powered by non-petroleum-based fuels by 1997, and 75 percent by model-year 2001.