What does the manual say?
Typically, the manual states what grade fuel to use just like it states what type of oil to put in, what other lubricants to use, etc etc. So it always amazes me to see people picky about what grade oil to use in their car, and yet resort to internet myths to determine what fuel to use.
Octane does not create power, nor does it improve fuel economy by itself. Octane allows the engine to operate to peak performance and achieve max MPG and HP...it in itself does not cause this.
So i refer you back to the manual. Those Ford engineers are some pretty smart guys. If running 93 octane would eek out some MPG, they would require it in order to bring their federally mandated CAFE number up. How is it worded exactly when it comes to what grade of gas to run?
If it says Premium Required, then you have no choice but to run 91+.
If it says Premium Recommended, then it means you could run 87 octane, as a knock sensor will change fuel and timing curves to run safe...but if you want max performance, you need 91+ for max HP and efficiency.
If it says you can run 87, then usually there is no benefit in running higher octane levels at all. (without mods or changes to the tune)
Octane is just the measure at which fuel resists burning. The lower the number, the easier and complete the fuel burns. That means lower octanes actually make more power. Depending on engine setup, compression ratios, timing and such, higher octanes are sometimes used to prevent pre-ignition/detonation. There is nothing "inferior" about 87 octane vs 93 octane. So no need to feel like a cheapskate for not putting 93 in your car if 87 is all that is required
A 1MPG difference can be attributed to a lot of factors. There are way too many real world variables to accurately measure a vehicles MPG by using the odometer and pump scale.
