I live in northern NM where we experience warm summers and cold winters. I keep a running spreadsheet to track fuel economy and definitely see the effect of the seasons.
A few things to note on the graph below:
-I got the car in late summer, 2012 and it took about nine months to learn how to drive it to get decent fuel economy. It did not help that this learning curve occurred over the winter where the fuel economy suffers due to the effects of the cold.
-I got another Insight about two years ago and used that car more than the one that I track with this spreadsheet, so the data for the last two years is a bit more sporadic. I did start using this car more recently.
-There are always outlier full-tank MPG readings, like for road trips, weird weather, new tires, etc.
-The red line (running average of full tank MPG results) has settled in a bit above 71 MPG for my use of the car.