Use the Octane recommended in the owner's manual for the vehicle. Using a lower octane can result in engine damage, possibly serious engine damage. Using lower octane, if you encounter detonation/pre-ignition would cause the PCM to retard the ignition timing and that could result in worse mileage.
Higher Octane fuels, depending one the area you live and the EPA mandated blend of fuel you must have, may have less oxygenated additives in it, that might result in a tiny gain in mileage. Some areas may have the same oxygenating additives as the low octane, and you'll NOT see any gain in mileage. Even if you do see a gain in mileage, its NOT likely to offset the higher cost of the higher octane fuel.
Octane is merely a measure of the burning properties of the fuel, it has nothing to do with the energy density of the fuel, all octanes "should" produce the same amount of energy. I mentioned before, because of various factors and mandated additives, you might find in some areas a barely measureable difference in energy density between octanes. Higher Octane fuels only resist pre-ignition/detonation better than low octane fuels, so if you build a motor with compression and tune to push harder and make more power, it will be more likely to detonate/pre-ignition and thus require the higher octane fuel. The cause for making more power is the motor design, the effect is higher octane, its NOT the other way around.
AFA, headers, custom exhaust and higher performance Catalytic Converters, NOT the way to go, the OEM exhaust is already very free flowing, its doubtful you're going to see any mileage increase by trying to modify it, and then enough to offset the high cost of installing a custom exhaust, but hey, at least it will be more prone to failure and leaks. I'd stick with the other suggestions listed so far. A good tune-up, higher pressure in the tires and mindful driving habits are your best course toward better mileage. The best mod, in my guess, would be one of the programmers that changes the PCM tune, that might produce actual results for the money spent.
The fans? Hmmm, I was under the impression the reason they went with an engine driven fan OEM is because there wasn't room to fit electric fans, perhaps I'm wrong or you can find aftermarket fans that fit?
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