U.S. drivers are pumping billions of dollars of premium gasoline into cars that may not need it, according to UC Davis researchers, in a survey believed to be the most detailed to date. Demand for higher octane gasolines -- available as premium and midgrade at 10 to 20 cents a gallon more than regular -- has steadily increased until it accounts for about one-third of the total U.S. gasoline market. "It amounts to billions of wasted dollars," says Dan Sperling, director of the Institute of Transportation Studies at UC Davis. "It is clear that people believe that premium gasoline provides a lot more benefit than it really does." Even industry analysts agree that people are overbuying. Oil and auto industry tests suggest that only 5 to 15 percent of new passenger cars actually need higher octane fuels. The UC Davis study found that many people buy premium gas for vague and unsubstantiated reasons: to improve performance and fuel economy and to improve reliability -- as insurance of sorts on a very large household investment. Women drivers are more willing to buy premium gasoline than men. New Yorkers are more likely to buy the more expensive fuel than Californians. Also, owners of fuel-injected cars prefer premium. The price has a big effect, the researchers concluded. When gas prices rose during the Iraqi invasion and occupation of Kuwait two years ago, sales of premium gas went down.