I always have used Premium (particularly Exxon, but I doubt there's a different in brand) in all our cars, and my wife (who drives like an 'economy' driver) gets 27 mpg on her '93 commuting to work (mostly highway, a little bit secondary roads).
On the other hand, I'm lucky to get 23-25 on my '93 (I drive a little -- oh heck, a lot -- more "con brio"). Both cars, by the way, have wide, sticky V-rated, tires (205/55-15 and -16's, respectively), which aren't good for fuel economy.
And on my '84, which I use mainly for "around town" errands, gets only an appaling 19-20 mpg (with more fuel-economical, harder H-rated, 185/70-14 tires).
The above figures are for summer tires, however -- take about 4mpg off all the cars when I put on their studded snow tires for winter, and for the effects of increased alcohol (oxygenation) additives "to reduce ozone" in this region.

The reason for using Premium (93 octane), however, is that all these engines are designed for at least 91 octane fuel (per your owners manual). If I could buy 91 as a mid-grade, I would use that instead, but around here mid-grade for any brand is a mere 89. Under acceleration, a knock sensor would retard the ignition timing, and would result in less power and less fuel economy, IMHO.
I don't mind a few cents more to be sure.