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By Jarrett Renshaw NEW YORK, Sept 20 (Reuters) - U.S. consumers wasted billionsof dollars last year filling their cars with costlypremium-grade gasoline for no tangible benefit, according to astudy by the country's leading motorist advocacy group. The report by Heathrow, Florida-based AAA comes as low pumpprices and a growing economy enticed U.S. motorists to buy morepremium-grade gasoline in June than in any month since 2003,according to the U.S. Energy Information Administration. While an overwhelming majority of cars on U.S. roadways aredesigned to consume regular gasoline, the study found thatmotorists still use premium-grade in the hopes of achieving morehorsepower and better fuel economy. As a result, U.S. driversspent an estimated $2.1 billion extra by using premium gasolinein vehicles designed for regular fuel. Premium provides little or no benefit to cars designed torun on lower octane, regular gasoline, the study said. "Drivers see the 'premium' name at the pump and may assumethe fuel is better for their vehicle," John Nielsen, AAA'smanaging director of automotive engineering and repair, said ina statement. "AAA cautions drivers that premium gasoline is higheroctane, not higher quality, and urges drivers to follow theowner's manual recommendations for their vehicle's fuel." What distinguishes premium from other grades is itsincreased ability - expressed in octane - to resist prematuredetonation, or knocking, as it is compressed in the enginebefore ignition. Premium gas is used in high-compression engines so it isoften associated with high-performance sportscars or luxuryvehicles. Last week, a gallon of premium gasoline was selling onaverage 48 cents higher than regular gasoline, among the widestspreads since 1994, EIA data shows. Refiners such as Royal Dutch Shell have focused onpremium gasoline sales due to healthy margins and its risingpopularity. In June, 45.3 million gallons of premium gasoline was sold,the most in any month since 2003, according to EIA. Sales ofpremium gasoline, which make up less than 15 percent of U.S.gasoline sales, have increased at a faster rate than sales ofregular gasoline in recent years. AAA tested 87-octane (regular) and 93-octane (premium)gasoline in vehicles with a V-8, V-6 or I4 engine designed forregular-grade fuel. The testing found no significant increasesin horsepower and fuel efficiency. The survey found that 70 percent of U.S. drivers own a carrequiring regular gasoline, while 16 percent drive cars thatrequire premium fuel. The remaining 14 percent require mid-gradegasoline or an alternative source. (Editing by Bernadette Baum)