I know most folks don't keep track of their mileage but i do. it varies with each tank from 4 to 5 miles per gallon. i think it depends on the percentage of ethanol. used to be if i used premium i got better mileage because the knock detector adjusted the ignition timing for higher octane. now the mileage is the same or worse. ... .
I am one of those that check my mileage every time I fill up. The only time I have any variance of the magnitude you described is highway vs local. That said I usually average high 13 to 14 mpg per local fill up using no name 93 non E. You can subtract 1 mpg when I run high test E gas. 92 and above is the manufacturer's recommendation for my engine. I also try to use the same station.
Ron