I am thinking that average speed per tank might reveal a lot for you in this analysis, though it strikes me that it would be problematic to determine - perhaps activate a timer every time you start/stop the engine for the duration of every fill-up. Living in the Hampton area, I am guessing you might have to deal with traffic congestion at times (?)
As Kev M noted, I notice a significant decrease in my mileage in the winter months - I understand it is to be expected, it just seems a little more pronounced in the Baja than with other vehicles I have or did have, like roughly a 10-15 % decrease during daily commuting.
The other consideration is tires - I have seen considerable changes in mpg in some of my vehicles when I have purchased new tires that differed from the previous ones.
109 K in 4 years! Wow! My 06, purchased in 9/06, just turned 25K!