car mileage, page-40

Octane is a measure of a fuel's ability to resist a phenomenon known as knocking, which a driver may detect as a pinging or knocking noise coming from the engine. Knocking is the explosive burning or uncontrolled combustion of fuel in an engines combustion chamber that can very quickly destroy an engine.

The higher the fuel's octane number, the more resistant it will be to knock.

Higher-octane fuels allow engine manufacturers to design more powerful and fuel-efficient engines.

Australia's current standard fuel is 91 Research Octane Number (RON) unleaded and most cars sold here since 1986 were built to accept it. However an increasing number of vehicles require 95 RON Premium fuel, while some high performance models require high-octane 98 RON fuel.

An engine's octane requirement is usually outlined in the vehicle's handbook. The fuel you select for your car must have an octane number at least equal to that specified. However a fuel with a higher octane number than required will not necessarily produce noticeable benefits, in spite of what some fuel company advertising suggests.

Some vehicles will definitely perform better on high-octane fuels and others will show no obvious benefit. Older vehicles with carburettors are unlikely to benefit unless they are specially tuned for it and even then any gains are likely to be limited. (Note: many cars built pre-1986 designed for leaded fuels will require high octane fuels.)

Some older models (i.e. vintage cars) with very low compression ratios may be incompatible with high octane fuels and could develop running problems.

The benefits for later fuel injected vehicles will depend on the sophistication of the car's engine management system and whether it can make use of the extra octane. Systems that are capable of adapting fuel and spark maps to the fuel being used are most likely to show improved performance while cars with less smart systems typically won't benefit.

The only way to find out is to fill the tank with one of the premium fuels and try it, although such trials are somewhat subjective, being based on a seat of the pants feel for the vehicle.

Anecdotal reports suggest that some cars will actually produce better fuel consumption on premium fuels, although in most cases this alone is unlikely to offset the increased fuel costs.

Ultimately it is up to the user to determine if the benefits justify the higher fuel costs.

Source: http://www.racq.com.au/motoring/cars/car_advice/car_fact_sheets/high_octane_fuels