I run 85 Octane in all my vehicles, as I live at 5000 ft...except for a turbocharged Escape, where I stick with the manufacturer recommended 87 minimum. I track every tankful of every vehicle I've ever owned for mpg. I found NO mpg correlation between 85 or 87 or 88 or 89 Octanes. What I did find was weather-related coorelation, as well as the obvious idle-times and load-related coorelations. Plus, how do you know the pumps are giving you accurate delivery volume measurements? I don't believe they are calibrated that well from station to station....or, even if using the same pump, can't the delivery calibration drift? Obviously, my experiences really don't mean anything, as the variables are too hard to control. Similarly, one other person's experiences is just another anecdotal scrap of evidence, which doesn't really have significant meaning. Especially when people find they get a very small difference in mpg, I don't place any value in their report. You need a calibrated volume of fuel, on a prescribed course, with the same vehicle, under the same driving conditions (weather, wind, barometric pressure), using the same tires (pre-heated, so, the first lap doesn't start on cold tires), at the same temperatures, with the same driver...and switch back and forth between different fuels, before I'd believe any small changes in efficiency. Then, you'd have to change the driver, and get the same small difference in efficiency. Only then would I believe a 1, 2, or 3% difference. And, that's just for that particular driving course and speeds. Super-or Turbo-charged vehicles are able to get pretty darn good efficiency on very low octane fuels, as long as the boost is kept minimal. But, when that boost comes on, in order to take full advantage of the boost, many of these systems REQUIRE higher and higher Octane levels to avoid pre-detonation. So, again, it depends upon the load requirements as to whether there is better performance, or, better efficiency, or both...or neither...depending upon blah-blah-blah.
However, can't significant detonation easily be heard in smaller engines with stock mufflers? I know in engines I've had in the past, not only could I hear it, but, I could feel the power output wane. If that is true, and I'm not positive that it is true, as long as you don't hear it/feel it, your Octane level isn't too low for those specific driving conditions.