2015 C-Max Owner's Manual states: "Octane Recommendations: We (Ford) recommend regular unleaded gasoline with a pump (R+M)/2 octane rating of 87. Some stations offer fuels posted as regular with an octane rating below 87, particularly in high altitude areas. We do not recommend fuels with an octane rating below 87. Premium fuel will provide improved performance and is recommended for severe duty usage such as trailer tow."
Assuming this is the same wording that Swamp45755 (see a couple of posts above) has in his 2018 C-Max (congrats on your purchase BTW) Owner's Manual, I interpret the statement as you only get an MPG benefit from premium fuel octane only if you drive hard, meaning the engine does sense potential knock and will retard the timing if necessary (when piezo knock sensors detect knock).
Thing is, it won't knock if you aren't driving hard and your engine then doesn't have to retard timing.
It is possible it will keep gradually advancing timing (for better MPG) until it senses knock, constantly "hunting" for that optimal timing adjustment as you drive. Premium gas would allow more advance of timing, giving us slightly better MPG. I'm not sure if timing advance is there, or if there is much of it, from the baseline engine map though.
Maybe I can figure out if I start seeing advance of timing using the FORScan (Ford-specific smartphone ELM327 OBDII) app.
It could be the Ford engineers actually run the engine on the baseline engine map, and only provide the freedom to retard engine timing (knock sensed), not potentially advance it gradually. Anybody know if the engine control software algorithms actually do advance timing beyond the stored look-up table engine map value?
Biggest fuel problem is ethanol usage. Ethanol is less energy dense than pure gasoline. Our C-Max's, like most Fords, can use up to 15% ethanol fuels, E15. (Can't use E85, the high-ethanol 85% extreme fuel out there.)
Typical gasoline sold has about 10% ethanol (E10), so we lose about 3% or so MPG (-1.5 MPG) just from the energy density problem with those blends.
Pure gasoline is better for MPG, if you can find it !!