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iTrader: (18)

Buying premium gas is like taking vitamins — you can't always feel the difference and yet you know it's the right thing to do. But as gas prices climb, paying the extra dime per gallon for premium is like adding insult to injury. Eventually, the thought is bound to jump into your head: do I really need to pop for premium?

Until about 15 years ago, if a car called for premium gas and you pumped in regular, the car began to knock and ping and even vibrate. But that was before they essentially put a laptop under the hood of the automobile, said Dr. Loren Beard, senior manager of Environmental and Energy Planning, for Daimler Chrysler. Now, sensors take readings and tune the engine as you drive by adjusting the timing for whatever fuel you put in the tank.

The result is that a car that calls for the midgrade gasoline will usually run on regular without knocking, Beard said. However, its performance will suffer slightly. How much? It will be perhaps a half-second slower going from zero to 60 mph.

Volvo cars call for "premium fuel [91 octane or better] for optimum performance and fuel economy," said Wayne Baldwin, product/segment manager S60/S80. "However, there is absolutely nothing wrong with using 87 octane as the knock sensors and engine management system 'protect' the engine from knocking."

Baldwin, a former rally driver who competed in SCCA Pro Rally events said that engines have changed a lot in the past 15 years. "Cars built before 1990 probably do not have knock sensors and many brands back then relied on high-compression ratios for the best performance. Today [performance comes from] electronically controlled spark curves, turbos, variable valve timing, supercharging and knock sensors."

Issues of performance aside, Baldwin said you should never use gasoline that causes your car to knock. "Constant knocking or detonation is a real bad thing for engines," he said.

When choosing what grade of gasoline to use, Steve Mazor, principle auto engineer for Auto Club of Southern California, said it is important to read the owner's manual carefully. The key is to figure out whether premium gasoline is "required" or "recommended." If it is recommended then a driver could opt to use a lower grade of gas, if they were willing to accept slightly reduced performance and fuel economy.

However, Mazor added, "We don't recommend that people switch down. Let's say you switch down to regular, and you have to accelerate to avoid an accident and it doesn't accelerate fast enough. The Auto Club can't be responsible for causing that situation."

Edmunds.com has a Volvo S40 in its fleet, so we consulted the owner's manual to see the exact phrasing in regard to fuel requirements. It said, "Volvo engines are designed for optimum performance on unleaded premium gasoline with an AKI (Anti Knock Index) of 91 or above. The minimum octane requirement is AKI 87." It appears that Volvo is making a recommendation for premium gas but is not requiring it.

In Edmunds.com's Forums debates abound over the pros and cons of using different fuel grades. One member even suggested there was only one type of gasoline, no difference — except for price — between regular and premium. Other members recommended using premium gas even if the manual called for regular. We put this question to Mazor and Beard.

Mazor: "All this does is do a very good job of draining your wallet. People used to put in a tank of premium to get 'the good stuff' to help their engines stay clean. But now they put detergents in all grades so it doesn't really get you anything."

Beard: "If you have car designed to run on 87 [octane], it doesn't help to run it on higher-octane-level gas. But there are several exceptions." He said that the 3.5-liter Chrysler engines are designed to run on midgrade gas (89 octane) and it allows them to advertise a certain peak horsepower. However, it will run well on regular gas. "The difference is very small," he said.

Interestingly, Mazor noted that at some gas stations, there are only two grades of gas. However, they blend the regular and premium at the pump to produce the midgrade gasoline. This allows them to have only two underground tanks for the gas storage.

In Edmunds' forums some drivers expressed concern about the quality of gas sold at independent gas stations and advised sticking to the so-called "name" brands of gasoline.

"Typically the only difference is the additive package they put in the gas," Beard said. The additive package is often put into the gas as the tanker is filled up at the refinery. A common additive is a detergent agent. "The law requires a certain level of detergents in gasoline. Shell for example is putting in more detergent — whether that has a measurable effect to the driver is debatable."

Detergents have a marked effect on engine deposits. "If you take apart a modern engine that has been running on a modern fuel, and compare this to an old engine that was running on old gas, you can see an obvious difference," Mazor said.

The biggest difference between today's gas and the gas sold 15 years ago is the removal of lead. Taking out the lead, and developing effective catalytic converters to more completely burn emissions, have radically cut pollution.

While oil companies like to advertise the magical powers of their gasoline, it appears that there is very little difference between brands. Most drivers fill up at the nearest gas station or the one for which they carry a credit card.

Does a gas expert like Beard have a preference when buying gas? "I just watch the light on the dash. After it has been on for a day I get nervous and go to the closest station available."

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