Octane rating has no direct correlation between performance or gas mileage. It does however correlate to increased knock resistance, which is needed in higher compression vehicles. Knock (aka preignition) is when the fuel/air combo spontaneously ignites in the combustion chamber before the spark has intended it to. This causes the flame front to push down while the piston is going up. This can break things very fast depending on the severity of the preignition (how early it is). Most engines now have knock sensors that detect this and detune the motor before it can break things, usually by retarding timing. Higher octane means less knock, and your car (along with most imports) run a high compression ratio to make more power out of a smaller motor. Generally running regular wont hurt the car itself, especially if you never run it hard, as this is when your octane rating will come into effect. At low load the maximum air that can enter the engine is limited, and thus so is the fuel and the maximum pressure/temperature that is reached during a typical stroke.With all that said..... many "premium" gasoline mixtures ARE formulated for higher power output, but i doubt the differences are significant enough to warrant buying it for that alone. IMO if your car says to use it, i would use it, it could save your engine. My high performance vehicle will never see any other type of fuel.