When filling up at a gas station, the premium grade isn't always better for your car. Recent research by AAA found that higher octane fuel doesn't carry any added benefits for standard vehicles, but that many drivers occasionally use this kind of gasoline.

The study sought to determine how frequently drivers use premium gas when their vehicle is able to run on regular grade fuel. Researchers also tested the different types of gas to see if premium fuel offered any benefits in power, efficiency, or emissions reduction.

Gasoline in the United States is usually available in three different octane ratings. These include regular, midgrade, and premium.

The fuel-air mixture created by premium gas is able to sustain more compression within the engine, and this grade of fuel is often recommended for high performance and luxury models. AAA says these models make up less than 20 percent of the vehicles on the road in the United States.

In a partnership with the Automobile Club of Southern California's Automotive Research Center, AAA tested regular gas with an octane rating of 87 and premium gas with an octane rating of 93. These fuels were used in vehicles with V-8, V-6, and I4 engines that were designed to use regular grade fuel.

Researchers ran the vehicles on a dynamometer, a treadmill-like testing device that can measure horsepower, fuel economy, and tailpipe emissions. The vehicles showed no significant improvement in any of these areas when using premium gas instead of regular.

"AAA's tests reveal that there is no benefit to using premium gasoline in a vehicle that requires regular fuel," said Megan McKernan, manager of the Automobile Club of Southern California's Automotive Research Center. "Premium gasoline is specifically formulated to be compatible with specific types of engine designs, and most vehicles cannot take advantage of the higher octane rating."

In addition to the testing, AAA issued a survey to 1,011 American adults to determine their habits at the pump. Seventy percent owned a vehicle that required fuel, while only 16 percent owned a vehicle that required premium fuel. Ten percent owned a vehicle that needed midgrade gas, while 4 percent had a vehicle that used a source of alternate energy.

Eleven percent of the respondents whose vehicles required regular fuel said they had filled up with premium gas at least once in the past 12 weeks. On average, these drivers switched to premium at least once a month.

AAA used this data—as well as information from the Federal Highway Administration, comparative prices between regular and premium gas, and the average number of fill-ups—to calculate how frequently Americans get a tank of premium gas without needing to. They determined that 16.5 million drivers have unnecessarily used premium fuel in the past year, wasting a total of $2.1 billion.

"Drivers see the 'premium' name at the pump and may assume the fuel is better for their vehicle," said John Nielsen, managing director of AAA 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 use."

The manufacturer will recommend what grade of fuel to use in a specific model. If the manufacturer recommends premium fuel and a driver uses regular fuel instead, the fuel-air mixture will not be able to compress as efficiently. This condition can lead to knocking or pinging, reduced performance, and lower fuel economy. AAA did not test the effects of using regular fuel in a model that requires premium gas.

One common misconception is that an occasional fill-up with premium gas can help remove carbon deposits in the engine. However, lower grade fuels can still provide this benefit if they have detergent additives.

Another study by AAA earlier this year found that gas from retailers meeting the Top Tier designation can result in 19 times fewer deposits. This designation, which was established to create more stringent standards for engine cleanliness, is supported by eight automakers and provided by 46 retailers.