After my "freeway mileage run" I was looking at the fillup/mileage history and noticed one week during which I had much higher mileage than the others: 13.9 MPG as opposed to the generally around 11.8 MPG. Traced it to an "emergency" fill-up at a Shell station instead of the usual (rather lower-priced) Arcos.
Running the numbers to get miles/dollar, I bracketed the Shell fill by Arco fills because of changing prices, I got for three consecutive weeks: (arco $3.31/gal) 3.54 mi/$ -- (shell $3.47/gal) 4.01 mi/$ -- (arco $3.57/gal) 3.28 mi/$.
I didn't expect to see an 18% increase in miles/dollar across brands, but there it is. The more expensive gas might actually provide better economy. More tests are in order, and LoM will start feeding me numbers for Ealabhan, which she is driving now.
Try to watch, if you can determine it, the alcohol content of your fuel.
Ten percent ethanol may run cooler and have combustion advantages, but its energy content is lower than the higher octane gasoline.