Some have mentioned octane, octane and MPG are not related, just FYI.
87 octane and 95 octane have the same "energy" per gallon so will result in the same MPG.
Higher octane fuel is less likely to pre-ignite under compression than lower octane.
If your engine doesn't "knock/ping" on regular gas then running with anything else is a waste of money.
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For older engines that did not yet employ knock sensors, timing is not affected by pre-ignition due to low octane fuel, and as such, these engines are prone to knock on low octane fuel in certain driving situations - and when this happens, your MPG is negatively affected.
More recent engines that employ knock sensors adjust timing when pre-ignition begins, retarding timing to prevent hard knocks. Timing that is retarded from ideal happens a little too late for the engine to gain as much rotational energy from the flame dispersion, so MPG is negatively affected.
Energy content of gasoline is about equal, regardless of octane level, that is true. Getting the engine to convert this energy content into rotational power efficiently is the issue.
I will agree if someone has driving characteristics that would never cause an early engine w/o knock sensors to knock on low octane fuel, or with a newer engine w/knock sensors to retard timing due to pre-ignition, that person would not gain any MPG benefit from a higher octane fuel.
The major question is whether the added cost of the higher octane fuel is justified by having more than enough offset in better MPG, so that the cost of fuel per mile is lower. I personally think MPG is improved slightly with a higher octane fuel, but is not enough to lower the fuel cost per mile to justify it.