Americans wasted over $2 billion last year putting expensive premium gasoline into cars that don't need it, the automobile ownership group AAA said Tuesday.

Most car engines in America are designed to run on regular gasoline and, despite the superior-sounding name, premium gasoline provides no benefit over cheaper regular gasoline in those engines.

Premium has a higher octane rating than regular gas. That means it's able to tolerate higher pressures inside the engine. Engines designed to run on regular don't operate at pressures high enough to require the extra octane, so putting premium fuel into an engine that doesn't require it is simply a waste of money, AAA said.

"Drivers see the 'premium' name at the pump and may assume the fuel is better for their vehicle," said John Nielsen, AAA's managing director of Automotive Engineering and Repair. "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% of vehicles now on the road require only regular fuel, AAA said. Still, about 16.5 million drivers unnecessarily filled up with premium at least once last year, according to AAA. That means Americans spent more $2.1 billion on more expensive fuel for no reason, the group calculated.

On average, premium costs about 50 cents a gallon more than regular, AAA said.

Related: World's fastest VW Beetle goes 205 MPH

About 16% of Americans drive vehicles that actually do require premium fuel, though. These are mostly high-performance or luxury models. Using cheaper regular fuel in these engines can cause damage

In tests conducted along with the Automobile Club of Southern California's Automotive Research Center, AAA ran regular and premium fuels in various different car engines designed to run on regular. Using premium fuels in these engines did not produce more horsepower, better fuel economy or lower tailpipe emissions.

In some vehicles, premium fuel is "recommended" but not required. Those vehicles may operate with lower power or efficiency using regular fuel.

Still, it may not hurt the engine to use regular in these vehicles provided that no engine "knock" is heard, AAA said. If an unusual knocking or banging sound is heard from the engine, as it might be under particularly heavy use, premium fuel should be added to the tank as soon as possible, Greg Brannon, AAA's head of Automotive Engineering, said.

As always, he said, drivers should consult their owner's manual about which type of fuel is best.

Photos: Coolest cars for $18,000

A separate AAA study released earlier this year showed that cheaper off-brand gasolines can harm engine performance in the long term. This is a separate distinction from premium vs. regular gasoline, though. In that study, AAA was comparing gasoline sold by different retailers.

AAA found that gasolines sold by "no-name" gas stations -- ones that weren't selling a major brand like Exxon Mobil, BP, Shell or Chevron -- could cause serious build-ups of engine deposits over just a few thousand miles.

"Such carbon deposits are known to reduce fuel economy, increase emissions and negatively impact vehicle performance, particularly on newer vehicles," AAA said.

This can happen because the cheaper brands of gasoline may not use engine-cleaning detergent additives found in major name-brand gasolines.

Editor's note: Correcting typo in Greg Bannon's name.