More than half of Maricopa County's alternative-fuel vehicles purchased especially to help clean up the Valley's air seldom or never use clean-burning fuels, an Arizona Republic newspaper analysis on Feb. 19 shows.The continuing reliance on gas is raising questions about the county's efforts to use alternative fuel, part of a strategy considered essential to the region's battle against air pollution.For the past several years, the federal and state governments have pressured local authorities to beef up their fleets with alternative-fuel vehicles. The county, along with Valley cities, rushed to convert cars and trucks and buy as many new "eco-friendly" vehicles as they could afford.There's no legal requirement that government cars and trucks use the cleaner-burning fuel.The county tracks the fuel usage for 239 alternative-fuel vehicles, mostly dual-fuel ones that can use gasoline, propane, or compressed natural gas. Of those, 130, or 54 percent, used an alternative fuel 10 percent of the time or less last year. In fact, 47 of the alternative-fuel vehicles never filled up with an alternative fuel in fiscal 2003, according to The Republic's analysis of county data.Maricopa County operates nine stations that offer alternative fuel.The county has 268 alternative-fuel vehicles, including 22 that use credit cards to buy fuel and seven that are supposed to use methanol, which is not available. About 13 percent of the 2,000-vehicle county fleet runs on alternative fuel.In Mesa, about 70 percent of the city's 1,300-vehicle fleet uses compressed natural gas. The city has an additional 500 vehicles that are not eligible for alternative fuel, such as fire trucks or backhoes. But the Fleet Support Department consistently monitors use and prepares reports for the city manager and department heads to keep tabs, according to the report in The Republic.Within its alternative-fuel fleet, Mesa used natural gas about 75 percent of the time during December, according to the latest city statistics.