It’s a decision drivers face every time they fill their cars up at the gas station.

But if you’re like many drivers, you may have decided years ago to bypass premium and buy the cheaper regular unleaded gasoline.

But have you ever asked the question — what exactly is the difference between the two octanes?

“Premium is, yes, a few octane points higher, which provides a more efficient burn in the combustion chamber,” says Bill Griffin, owner of Griffin’s Neighborhood Auto Clinic in Farmington, Michigan. “But it is a choice. Slightly better fuel economy is there, but it’s not worth the huge price gap from regular to premium.”

Most gas stations offer three octane levels: regular (about 87), mid-grade (about 89) and premium (91 to 93).

Some gas stations may offer up to five different octane ratings, including a super premium, which typically has a rating of 93. Other gas stations may call their mid-grade “plus” or “special” and their premium “super.” If you’re unsure based on the description, check the octane level.

An octane rating, according to Exxon Mobile, measures the fuel’s ability to resist engine knocking, or pinging. The higher the octane, the greater resistance the fuel has to pinging during combustion.

Regular unleaded gas typically costs about 15 to 20 cents less per gallon than premium. That can lead to a savings of $100 or more per year, depending on how often you fill up.

In regards to the effectiveness of regular gas, the Federal Trade Commission says there are no advantages to using premium gas in cars that don’t require it. The FTC states that using a higher octane on cars that don’t require it will not lead to better gas mileage or to the car running cleaner or faster.

Your car owner’s manual lists the recommended or required gas.

“I myself feel that the auto industry has evolved to a point where regular octane fuel is fine, due to better quality and integrity of the fuel systems today,” Griffin says. “I only run regular fuel and never have had an issue.”

Phillip Reed, a senior consumer advice editor for Edmunds.com, a car shopping website, says with advances in technology, all grades of gas typically have plenty of additives to protect the engine.

“Today, engine control systems can compensate for low octane by monitoring knock activity and adjusting ignition advance to avoid knocking,” Reed writes in an article on why most drivers should stop using premium gas. “This sophisticated electronic capability effectively tunes the engine on the fly and gives drivers more flexibility in the grade of fuels that they can safely use.”

Mid-grade fuel lies somewhere in between the other two grades, typically with an octane level of 89. The Federal Trade Commission says if your engine in pinging with regular gas, then upgrading to mid-grade may stop it.

Only a handful of cars require or recommend mid-grade.

“On engines that do not require the higher octane, the benefits are there but less apparent,” Maas adds. “You could gain a slight bump in fuel economy. This is because the flame front [or the area of combustion] is slightly slower and gives a little more power during the power stroke.”

Although states may have differing regulations regarding what octane rating constitutes premium, it typically starts at about 91.

Some luxury brands, such as BMW and Mercedes, have high-compression engines and require the use of premium gas to prevent the engine from knocking. In cases where the manufacturer recommends premium gas, auto experts say using regular gas is typically fine, unless the manufacturer requires premium.

Auto experts say not using premium in cars that require it can result in the engine knocking, which can eventually decrease the engine’s efficiency. The FTC reports that light knocking typically won’t cause any engine damage, although heavier pinging can.

“Many people think that premium gas is recommended because of all the additives that involve keeping the internals of the engine clean,” says Eric Maas, owner of Maas Auto in La Grange, Illinois. “While the cleaners are good for the engine, that is not true. It’s the higher octane. Higher-compression engines need the higher octane so that the fuel doesn’t pre-ignite in the combustion process and damage the motor. Octane slows and controls the detonation of the fuel during the compression and detonation of the fuel.”

Editor's note: This is an updated version of an article originally posted on Oct. 11, 2013.