| I have heard three things about gas:
1) If the manufacture suggests using 87 octane, then, using 89 or 91+ octane gas has not been in the design considerations. The valve train, ignition timing, compression ratio, air intake, exhaust, temperature and density, motor oil, even gas spray pattern, etc. all plays a role in the engine function. If it is designed to use higher octane, the engineers will optimize all the factors. This is why the similar Toyota 3.5-liter V6 engine has different output between Camry and ES350 (87 versus 91+ octane).
2) If an engine were new and used higher-octane fuel, the engine metal/alloy would set wrong. So when the engine is older (say 80K or 100K miles later), the engine will not be able to use 87 octane without any 'knocking''. So, do not waste money on 'improve the engine' by higher-octane fuel.
3) The oxygen generator, i.e., ethanol, has less energy. The octane 89 fuel has no additional total energy per gallon. The only difference is we may get the 1% to 2% less loss between 87 and 89 octane. The 91 and above can only has 2 to 3% less lost. It is too little to make any significant difference.
The engineers know much more than we do in the engine/fuel technologies. On the other hand, I have been told and experienced to slow down, smooth start and stop will improve the mpg.
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