I've seen similar FE changes. On a shortish drive (10 miles or so) just a few "small" differences do have a large effect. Even on my 33 mile morning commute, at 30deg I get 36ish mpg; at 40deg 39, at 50+ I am over 40. Initial conditions in those 1st 2 miles before I hit the highway play a dominant role, even with 30 highway miles to pad it.