Does premium gasoline get better mileage?

A Lexus dealer told me that premium gas required in their cars actually is NOT more expensive than buying regular gas because premium burns better and therefore gets better mpg, thereby negating the extra expense in buying premium. Is this true?

S. Campbell, Austin

In a car that recommends regular gasoline, premium will not get you noticeably better gas mileage. The only advantage is a lighter wallet or pocketbook to carry around. Inversely, when regular gasoline is used in a car that requires premium, the engine’s computer changes its operation — it retards ignition timing — to adjust for lower octane fuel. This change in operation can decrease gas mileage and power.

According to the American Petroleum Institute, premium traditionally has a slightly higher energy content, which can provide better gas mileage, but any gains will be difficult to detect in daily driving. If there is a mileage advantage, it will be a far cry from saying premium is cost effective and negates the extra cost.

The main difference between regular, mid-grade and premium gas is the octane rating, which is a measure of gasoline’s resistance to combustion. It’s commonly sold in 87, 89 and 93 grades; 91 or 92 are available in some areas. Certain engines (especially ones with a supercharger or turbocharger) need the high octane go-go juice to prevent engine pinging or knocking, which can be harmful to an engine.

Before you make your car shopping decision, though, remember that a car will run best on its recommended fuel. If the car requires (not recommends) premium, definitely use it unless only a lower grade is available. You should always use the gasoline that the automaker — not the dealer — recommends. That’s why it’s always phrased either “recommended” or “required.” If you don’t know what fuel to use, check your owner’s manual.

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Answered by Joe Bruzek on July 3, 2007 in I'm Just Wondering | Permalink

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