They switch gas formulas soon in my area.
Which formulation gives better mileage?
I would guess summer, but that’s only a guess.
People usually report lower fuel mileage with the winter blends.
I notice a ~10% drop in mileage in the winter. For instance my 2000 Blazer mpg is 16.5 in the winter and ~18 in the summer. My 2010 Cobalt under the same conditions get ~27 winter and 30 mpg in the summer. S. Jersey uses E10 year round, I’m not sure of the differences between the summer and winter blends.
Note: MPG is calculated every third or fourth fill-up (~1k interval).
Ed B.