Regular or premium? Psst, here' s a secret

Q: Does it matter whether I fill my car with regular, medium-grade or premium gas?

A: Probably not. Experts say most cars on the road will run just fine on regular — even if the owners manual says a car "requires" premium gas.

"For the majority of cars out there, their engine was designed to run on 87 octane," the octane rating of most regular gas, said Patrick Kelly, fuels associate at the American Petroleum Institute.

It is true that some high-performance cars are designed to run their best on premium high-octane fuel. The manuals of many such sports cars or vehicles with "super-charged" or "turbo-charged" engines say they require premium gas.

"Generally speaking, those vehicles will run fine on regular gas," said Geoff Sundstrom, a spokesman for AAA.

They just won't run as well as they would on higher octane fuels.

Does that mean a Honda Civic will accelerate like a Corvette when fueled with premium?

"Not really," says Kelly.

In fact, unless you own a high-performance car, you're unlikely to notice any difference between different grades of gas.

"In theory it does (boost performance), but the difference would be minuscule or hard to notice by any average driver," Kelly said.

It sounds counterintuitive, but higher octane levels prevent an engine's gasoline-air mixture from spontaneously combusting before the spark plug fires. High-performance engines compress the gas-air mixture more than regular engines, giving the tiny explosions that drive pistons more force. But if a lower octane fuel is used, the mixture will sometimes explode before it has been completely compressed, delivering less power to the engine or, in some cases, working against the other pistons.

In older cars, this caused "knocking," which was just the sound of pistons firing prematurely and out of sequence. Newer cars are designed to detect and compensate for premature piston firing, preventing the knocking noise, Kelly said. But their pistons can still fire prematurely, preventing engines from running at maximum power, he said.

Older cars also sometimes knock due to carbon build-up in the piston chambers, which reduces the chambers' size, causing premature firing. Detergent gasoline additives required since the mid-1990s have solved that problem in many cars by keeping engines cleaner, Kelly said.

While premium gas won't help most cars run perceptibly better, high octane fuel has another benefit: "You get a slight advantage in fuel economy," Sundstrom said.

But that advantage is usually more than offset by the fuel's higher cost.

"For consumers that are worried about the high cost of gasoline, they're going to do just fine purchasing the regular grade of gasoline," Sundstrom said.

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