About 9 percent of all gasoline sold in the U.S. in May 2009 was premium grade, according to the Energy Information Administration. "Consumer Reports" magazine says, "If owner’s manual says to use regular fuel, do so—there's no advantage to a higher grade." The "Associated Press" said premium gas—which has a higher octane rating and costs more per gallon than regular unleaded—should be used only if the manufacturer says it is "required". Cars with turbocharged engines and high compression ratios often specify premium gas because higher octane fuels reduce the incidence of "knock", or fuel pre-detonation. The price of gas varies considerably between the summer and winter months.