[kach22i]

I too noticed a drop in mpg with the switch to winter gas this year, new to me car so cannot compare to other years. However, it was a bigger drop than I ever noticed in my Geo Tracker and Chevy S-10 going back 30 years in memory collectively.

I unfortunately used twice a Shell gas station that was selling something else than Shell, just as bad as other gas stations.

If your car(s) use premium like my two cars (45 old Porsche, 10 year old Infiniti) the nitrogen additives in Shell make just as much of a difference as between summer and winter blends of gasoline.