I hope this stimulates some useful thought. I have a 2010 Ecoboost that like most modern cars has the recco of high test to get max performance. A bit of experience seems to indicate that I get better gas mileage with lower grade fuel. I am aware of the computer and incipient detonation detection/regulation system in modern cars, as well as control of solid fuel injection, etc.

Brainstorming a little -- Hi-test fuel (here 91 octane) is intended to allow earliest ignition without detonation (ping or spark knock). That said, the probable effect is slower flame propagation which allows the piston to move a bit into the power stroke before max pressure occurs.

Brainstorming further, that effect is needed primarily when high power is called for - in which case at lower power a reduced flame prop rate may allow the flame and max pressure to affect the push to the piston less.

Lowest grade (87 octane) should have the fastest flame prop rate which should have the effect of best power at low (cruise-highway) power setting. As stated I think I am seeing this, with about 26 mpg on 91 and 28 mpg on 87. I plan to get more experience with it and try to confirm something.

I would appreciate thoughtful comments and especially any experience with or reference to any available hard copy. I would imagine there is info available in SAE but I gave up my membership in 1980 when I retired.

Thanks, Robert McCrory