Because it is user error, 99% of the time.
Shifting at low RPM will require more throttle input, and in turn more fuel.
Try 2 tanks shifting around 3,500rpm and see what happens.
When you mentioned the brand of tire you missed completely what I meant. Tire pressure should have been your first thought. Low pressure will absolutely drop MPG's.
Being it's later in the year you should be able to get away with running more pressure than Summer. I'd try around 35psi in all the tires, maybe a bit more if you're up to it.
Don't "think outside the box".
There is no box.
Do it correct, or do it twice.