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.