Vehicles that recommend the use of premium fuel can see a slight increase in fuel economy and performance, according to tests by AAA. However, the organization also notes that the increasing cost difference between regular fuel and premium fuel typically cancels out any savings a driver might see.

Premium fuel has a higher octane level and creates a fuel-air mixture which is better able to sustain engine compression. It is usually required for adequate performance in luxury and performance vehicles.

Researchers tested six vehicles with a variety of sizes, styles, fuel delivery systems, and air induction systems. Each vehicle recommended the use of premium fuel but did not require it.

Tests were conducted on a dynamometer, which measured the vehicle's fuel economy and horsepower. AAA says that since drivers are unlikely to experience any benefit from premium fuel in a standard city or highway driving scenario, the tests simulated extreme driving scenarios such as aggressive acceleration, carrying heavy cargo, and towing.

One of the vehicles in the test, a 2016 Audi A3, actually had a 1 percent decrease in fuel economy when using premium fuel. The 2016 Cadillac Escalade had a 7.1 percent boost in fuel economy. On average, the vehicles tested had a 2.7 percent improvement in fuel economy.

Premium fuel also tended to result in a modest improvement in horsepower, with an average increase of 1.4 percent. One vehicle, the 2016 Jeep Renegade, had a 0.3 percent drop in horsepower with premium fuel. The 2017 Ford Mustang saw the greatest improvement, with horsepower going up 3.2 percent.

Despite these benefits, AAA concluded that drivers were unlikely to save money even if their vehicle got better mileage with premium fuel. Researchers noted that while the average cost difference between regular and premium fuel was 10 percent or less from 2002 to 2014, it has varied from 20 percent to 30 percent since then. In 2016, premium fuel typically cost 50 cents more per gallon than regular fuel.

"There's no question that higher-octane premium fuel has the potential to boost a vehicle's fuel economy and performance. However, engines have to be calibrated to require that fuel to see the full benefit," said John Nielsen, AAA's managing director of automotive engineering and repair. "Based on AAA's testing, vehicles that only recommend premium gasoline can't take full advantage of higher octane fuel and, as a result, the benefit that comes from upgrading to premium gasoline may not offset its high cost."

In a separate study in 2016, AAA concluded that premium fuels did not provide any extra benefits for standard vehicles that did not recommend or require this type of gasoline. However, a survey of 1,011 drivers found that 11 percent of drivers had recently filled up with premium gas instead of regular, and that these drivers got a tank of premium fuel once a month on average.

AAA says there is a common misconception that an occasional fill-up with premium gas can help clean out the engine by removing carbon deposits. The organization says lower grade fuels will still have detergent additives that provide this benefit, and recommends using fuel from retailers with a Top Tier designation for the best results.

The more recent study says that while premium fuel does not offer any benefits from a cost perspective, it may be able to improve vehicle performance in some circumstances. Some drivers may be able to eliminate a knocking sound when accelerating under a heavy load, improve fuel economy when towing or carrying a heavy load, or briefly improve horsepower when driving a performance vehicle.

AAA says drivers should take their vehicle to a mechanic if a knocking sound does not go away with the use of higher octane fuel. Drivers can also see if their vehicle benefits from premium fuel use during towing or hauling by making comparable trips using different fuel grades and tracking fuel economy.