The energy content of gas is just about the same regardless of octane rating. There should be no noticeable gain in performance by using higher octane fuel unless the engine is sensing knocking and retarding the timing. In this case you are simply getting worse gas mileage by putting low octane fuel in and your engine is getting lower gas mileage than it should.
The octane rating is a rating that determines the gasoline's resistance to spontaneous ignition upon compression, and is the percentage of the hydrocarbon chain that is actual octane. Octane numbers below 100 are a measure of the octane in the fuel while number over 100 are considered a "performance rating" since it is not possible to have an octane percentage higher than 100 but it is possible to have a fuel rated at higher than 100 octane. Fuels rated over 100 are simply more resistant to spontaneous ignition upon compression.
The aspect of your engine that determines what octane you need to use is the compression ratio. High performance engines need high octane fuels because they are generally high compression engines and need a fuel that withstand the high compression without igniting without the spark plug causing it to or very rapidly burning causing and an uncontrolled explosion in your engine rather than a slower, more controlled burn.
The fact of the matter is that using low octane fuel does not necessarily decrease gas mileage and performance because it is lower octane, this may only be the case if the engine is sensing knocking and compensating with a different, less efficient timing. The fact is that using low octane fuel causes ENGINE DAMAGE because it causing knocking. Detonation is not something that is good for your engine, it puts massive amounts of stress on the internal components of the engine.
If you have a low compression engine, you may be able to get away with putting a lower octane fuel in it. However, if your engine is high compression, you must use high octane fuel, or cost yourself much more money through very premature engine failure. Do not be cheap with your gas if you have a high compression engine.
-Ali Al-Chalabi, using 93 octane.
[Edited by Ali Al-Chalabi on 03-02-2001 at 01:53 PM]