That's exactly what can happen in a modern, tiny turbo engine, that it advances the timings if it has higher octane fuel. That doesn't mean it works out from a cost:benefit ratio unless you care about those last few HP, which only matters if you're flooring it... which isn't someone too concerned about fuel economy or vehicle TCO.And the octane rating of the gasoline in and of itself will not affect the fuel economy. There are various ways to affect the octane rating of gasoline but "straight gasoline" with two different octane ratings has the same energy content. Only if your engine is able to compensate by advancing the timing will you get a small improvement in efficiency. But if it cannot do that then a higher octane rating will do nothing for efficiency. It doesn't burn better nor more completely.