I have also been very interested in how outside temps affect mpg. I have been collecting data for quite a while, but honestly not sure how to compile it to make since.
Daily I kept track of my mpg, engine temp when done, adv speed, outside temp and if the engine was cold, warm (heated via coolant heater) or hot (as in just ran and is already warmed up.
If someone wants to take a shot with the excel data feel free.
It is at http://www.uwgb.edu/nevermab/mpg.xls