The octane rating of gasoline relates to how difficult it is to "set it off", in a controlled manner, in your cylinder... The higher the number, the harder it is to "set off". If the compression in your engine is high, the likelihood of a premature ignition rises (it ignites on compression as opposed to the spark plug igniting it) and a higher octaned fuel should be used to prevent this pre-ignition combustion of the air/gas mixture (knocking). Octane does nothing for mileage...it just makes it (the gas) harder to ignite. The best gas to use, would be the lowest octane number that you can use that doesn't pre-ignite on compression (engine knock). I had to run high octane in 66 Chevelle, but, I was running 11.7 to 1 compression (milled and ported heads on a bored and balanced block). On my Hemi Commander, I use 87 octane. I've used 89 and 91 when towing as an experiment, even though I wasn't getting ignition knock on 87 and found absolutely no difference in performance or mileage, even when towing 6000 lbs through the Rockies. I use 87 now, all the time. Until I start getting pre-ignition, I find no reason to pay the "toll" for higher octane (anti-knock) gasoline.