Finally emptied my tank of 91.
Previous fill of 87 -- 429mi, 16.6gal (purchased at $3.87/gal, 91 at this time was $4.09/gal)
Emptied fill of 91 -- 452mi, 16.5gal (purchased at $3.97/gal, 87 at this time was $3.87/gal)
The 87 fill was emptied over a weekend -- plenty of highway runs and a few short local trips
The 91 fill ran about the same highway, but because of the higher MPG and the gas saved, it allowed me to go through 2 extra days of local work commute of about ~50mi.