http://www.cougarboard.com/board/message.html?id=613778

Cars that are designed to get maximum performance out of the engine often have higher compression ratios than more run-of-the-mill motors. Those higher compression rations generally require higher octane fuel, not because higher octane fuel is better than lower octane fuel, but because different octane fuels have different detonation points. It's never a matter of better or worse fuel, but of matching the octane of the fuel to the design of the engine.

Most modern cars have variable ignition timing so that the engine can adjust somewhat to variations in fuel. That's why you will often see people say that you can use a lower octane fuel than what is recommended for the car (or what the engine was designed for) as long as you don't hear that pinging noise that indicates premature detonation of the fuel in the cylinders. However, when the engine management system adjusts to control premature detonation, what you give up is peak performance or efficiency. The engine is not running as well as it could be.

This could be only a minor difference. A loss of 1 or 2 mpg or a very slight downgrade in horsepower. This loss is also negated somewhat at higher elevations since the density of the air plays a part in the process. Generally speaking, I like to use the octane rating of the car that my motor was designed for. However, an occasional tank of something lower is certainly not going to hurt anything.