What winter tires are you using? We have two Cruze TD's, we have Altimax Arctics and Nokians and the difference is pretty small.
I am curious, was the inflated diameter the same between summer and winter tires?
I see a noticeable difference in mpg on my car (2-3 mpg less in the winter) and have noticed that the inflated diameter is different by 0.5" and my speedometer goes from being right on to reading 2-3MPH faster than my ground speed (per the GPS on my phone).
I have a '97 Volvo V90 (RWD wagon with a straight 6). I get 18-19MPG in the winter and 19-21MPG in the summer (depending on how much I play with the skinny pedal).Good question I'll have to put my gps in it with the snows to check the speedo.
I know the Static loaded radius was nearly identical only a couple mm difference. the two tires that showed the 5.5-6 mpg difference.
What kind of car and what is the mileage do you normally get.