Originally Posted by Kabigon
1) The Focus PCM can determine the octane of the fuel (not in an absolute sense, but in a relative sense). Anyone with an IDS can monitor the PID Learned Relative Octane Adjustment. With Chevron 91, this number will usually max. out (max. value is 100). With Shell 91, it usually stays in the high 80's or low 90's (and will rarely hit 100). With lower octane fuels, this number will go down accordingly.
2) Getting back to the argument as to whether or not premium fuel will provide better performance in an engine that is specified to run on regular:
2013 Mustang GT (5.0L Ti-VCT V-8 engine)
Recommended fuel: unleaded regular
HP: regular fuel: 402 @ 6,500, premium fuel: 420 @ 6,500
Assuming that you're correct (which I think you are), thank you for helpful information!