The rule of thumb says that a car loses at least 10 percent of its value the moment it leaves the dealership. But soaring used-car values are turning that formula on its head for many fuel-efficient vehicles, which are suddenly in high demand as Americans fret about escalating gas prices and do not want to pay for a new car. Used cars over all are retaining a higher percentage of their original value than ever before, according to auto analysts who track prices. Compact cars that are one to five years old are worth, on average, about 30 percent more on the wholesale market now than just six months ago.