Already finding some interesting things. Consider the plot below:
This is 5-day average mileage over several months (date is on the x-axis as month/day). I corrected the data to get the mpg if the temperature was always 60 deg F with no wind.
If I had just been looking at the un-corrected data (red line) going into the fall and winter (Aug through Feb), I would have thought that I was getting worse. But, in reallity, my weather-corrected mileage was getting better that whole time!
Also, I could have looked at that period in late March and thought that somethin I'd done must have caused a big spike in mileage. In reality, it was just a brief spring warm-up that went away.