AAA: Don't waste money on 'recommended' high-octane gas

Testing showed an average 2.7-percent improvement in fuel economy and just 1.4-percent increase in horsepower.

Many drivers may be wasting their money on premium high-octane gasoline, according to an American Automobile Association study (PDF).

The research focused on vehicles that show premium octane gasoline as "recommended" but not "required" by the manufacturer. Authors attempted to measure any increase in horsepower or fuel efficiency, compared to the average price increase.

AAA tests achieved an average 2.7-percent improvement in fuel economy using premium gasoline. The fleet included the Cadillac Escalade (+7.0 percent), Ford F-150 (+5.0 percent), Ford Mustang (+2.3 percent), Jeep Renegade (+1.9 percent), Mazda Miata (+0.7 percent) and Audi A3 (-1.0 percent).

The more expensive fuel was associated with an average horsepower increase of just 1.4 percent, led by the Ford Mustang (+3.2 percent), F-150 (+2.1 percent) and Audi A3 (+2.1 percent).

The report does caution that drivers should always follow the "required" high-octane rating, otherwise the engine could experience damaging knock or ping from preignition under load.

"During road testing, the Audi A3 had audible engine knock in road driving on a severe grade at low speed when using regular gasoline," the study observed. "This supports using premium gasoline in the A3 at all times."

Overall, even the largest increases in power and fuel efficiency were far below the average price gap between regular gasoline and high-octane alternatives.

"Using national averages, the difference in retail cost between regular and premium gasoline is approximately 20 percent to 25 percent, or 50 cents per gallon," the AAA says.