Basically, the octane rating is a handy-dandy way of measuring how quickly the fuel burns. If 10 litres of 95 RON was run in an engine, next to an engine running 99 or 100 RON fuel.. the 95 RON would run out first, according to theory. The theory being the higher the rating, the slower and more controlled the fuel burns.
In real-life terms, this means that for x amount of engine revolutions, you'd need LESS of 99/100 RON fuel than you would 95. Simplified yet again, this means you get more movement for equal amounts of fuel, IF you use high-octane fuels.
100 RON fuels may give the impression they're loaded with C8H18, but the amount is actually found in very small quantities, along with paraffin, napthene and a load of hydro-carbons, (like Octane). As RobM said, Octane isn't the only pivot factor in the quality of fuel. The need for higher-octane fuels was realised when we took to the skies in WW1 and 2. Higher octane fuels could power aircraft at higher altitudes than lower alternatives.
As we all know, the higher the altitude, the thinner the air. This is bad for engines, let alone planes 10,000 feet up. Problems with air-fuel mixture here could equal fatal stalling. To counter this, turbochargers would put to good use in order to compress the air being forced into the combustion chambers.. to match the density of the air being combusted, more explosive grades of fuel were distilled.. cue the 100RON etc.
And thus ends the chemistry/physics lesson: moral of the story, higher C8H18 rating, more MPG.
