Can't say I've ever seen a consistent difference in gas mileage from one brand to another. And just about any petrochemical engineer down here in Texas will probably tell you the main differences are additive packages and, of course, octane ratings. However, some out-of-the-way convenience store with two gas pumps and very few customers may have some seriously old gas in their storage tanks.
And if your engine doesn't knock with regular, going to premium won't gain you anything. That's assuming you have an older vehicle without a knock sensor. But, man, that 100+ octane gas sure does smell nice! Air cooed two-stroke dirt bikes and older car & truck engines with 11:1 to 12:1 compression love it.
My 2016 Mustang GT requires premium gas to make its rated 435 hp, but I normally run regular. Tried a tank of premium once, but couldn't tell any difference in power or gas mileage. But it probably would have been faster down the drag strip at WOT.
Also tired some Exxon regular a while back and saw around 5% better mileage than I usually got on a tankful. However, the second tankful I bought from that same station gave me about the same mileage as usual. And I thought I was on to something.