When you're filling up at the gas pump, it's easy to assume that "premium" is the best. So, opting for a 93 octane fuel rather than a lowly 87 must help your car, right?

Not necessarily so, said Travis Mock, manager of AAA Auto Repair. He often hears people say, "My baby only gets the good stuff."

"Unless the manufacturer specifically recommends it, using premium fuel is a waste of money," said Mock, who has been in the auto-repair business for 20 years and with AAA Arizona for about a year.

On average, it costs more than 20 cents per gallon for premium gas over regular unleaded. That may not sound like much, but it adds up, and if the ignition system isn't hot enough to burn the premium, it will leave carbon deposits on top of the pistons. That will increase the compression, and eventually the vehicle will require premium unleaded.

Of course, certain high-performance cars require a higher octane. But most cars don't need it.

If you're using the wrong octane, sometimes you'll hear the engine "knocking," because the fuel and air mixture are combusting unevenly in the cylinder.

"They're like small speakers," Mock said. "They try to prevent pre-ignition. As the piston travels up, what's supposed to happen is air and fuel mix together. The spark ignites that mixture and forces the piston to travel back down inside of the engine."

While most cars have at least a little bit of knock, if the knock is audible, that can be cause for concern.

"You basically have two forces coming at each other versus working in tandem," Mock said. "Think of it as a little construction man inside of your engine with a hammer. He can only hit that piston so many times before it breaks."

Damage caused by too-low octane can run into the hundreds to thousands of dollars if ignored.

AAA Auto Repair answered some questions about the ins and outs of octane.

Q. Does premium gas give you better mileage?

A. It's not going to do anything for your fuel mileage. Just because you're burning a little extra octane, it doesn't mean your fuel economy is going to increase.

Q. What if you use regular unleaded for a premium car?

A. If the vehicle's computer detects a knock, it will start to back off the ignition timing, which will decrease the mileage slightly.

Q. So, will it void my warranty?

A. Keep in mind if the manufacturer says you should use premium unleaded gasoline, you should. Oftentimes, owners' manuals usually just recommend it, which means it usually doesn't affect your warranty.

Valerie Vinyard is a public affairs specialist for AAA Arizona. Contact her at vvinyard@arizona.aaa.com or 258-0518.