there wont be an mpg change unless your car is tuned for higher octane. chevron probably has the best long term fuel with its techron but like any detergent you wont be getting and extra mgg, actually it would be lower, but the difference would be miniscule.
otherwise with cheap arco/other gas that has no detergent you would be better off putting a bottle of quality fuel system cleaner through the engine ever few hundred to few thousand miles if longevity means anything to you. but like your asking, in theory fuel that is NOT california blend or has no detergent in it will get you the best possible mpg. if you never use a fuel system cleaner and like to go 100,000 miles on your factory fuel filter, your best off using chevron or shell though... you might get an extra 10k to 20k miles out of your vehicle before the motor blows at 150,000.
by comparison diesel fuel is much more filthy and lower quality than gas so diesel vehicles have much larger fuel filters with much more frequent recommended change periods. i replace my duramax on every 10,000 miles, and its bigger than almost all oil filters. most gas vehicles fuel filters are about the size of a small coffee cup while diesels have them as large as a big gulp 😛