Americans are enjoying some of the lowest gas prices in years, but they spent $2.1 billion more than they needed to by buying premium-grade gasoline when regular-grade fuel would do just as well, a national study from AAA found.

The national study found 16.5 million drivers used premium gasoline in a vehicle designed to run on regular fuel at least once in the past year. Drivers unnecssairly upgraded to higher octane fuel 270 million times even though the more expensive gas did not produce more horsepower, result in better fuel economy or produce fewer tailpipe emissions.

"Many drivers think they are giving their car a treat by occasionally filling up with premium-grade gasoline, even though their vehicle calls for regular unleaded," said Gail Weinholzer, a spokeswoman for AAA Minnesota-Iowa. "AAA cautions drivers that premium gasoline is higher octane, not higher quality, and urges drivers to follow the owner's manual recommendations for their vehicle's fuel."

About 70 percent cars on the road require regular unleaded gas while only 16 percent require premium fuel. About 10 percent of U.S. drivers own a vehicle that requires mid-grade gasoline while 4 percent run on an alternative energy source.

Weinholzer said motorists should follow the fuel recommendations spelled out in their owner's manual.

"Drivers looking to upgrade to a higher quality fuel for their vehicle should save their money and select a TOP TIER™ gasoline, not a higher-octane one," she said.

That claim is backed up by the Federal Trade Commission.

Octane ratings measure a gasoline's ability to resist engine knock — a rattling or pinging sound that results from premature ignition of the compressed fuel-air mixture in one or more cylinders. Most gas stations offer three octane grades: regular (usually 87 octane), mid-grade (usually 89 octane), and premium (usually 92 or 93), the commission says.

"Unless your engine is knocking, buying higher octane gasoline is a waste of money. Premium gas costs 15 to 20 cents per gallon more than regular. That can add up to $100 or more a year in extra costs." the commission said.