I've gone thru 7 winters in massachusetts, and drive the same commute each day, with little weekend travel. My commute is not highway, but I can go 45-50 most of the time, with few stops and slowdowns. Summer I get a consistant 30-31 mpg, winter consistant 27-28, like clockwork.
The cold effects in several ways. All fluids (oil and grease) are thicker so offer resistance. Tire pressure goes down, if you haven't checked it. My previous Eclipse with awd, in summer backing up out of the drive, clutch in, it would coast. In winter, definitely stop coasting much quicker.
Gas gets vaporized poorer so less efficient burn (regardless of electronics), engine warm up even if you start off rightaway, traffic slowdowns.
The change from summer to winter gas is mostly to ease vaporization, and I can't believe the little power difference would make much difference