What caused change in my avg MPG?

what does it really matter in the end?
Are you going to spend more $$ on lighter tires? - probably not, defeats the purpose of saving money
Will you make sure to drive only when there is no wind? - doubtful
will you change your driving habits? - again doubtful but how much would it save if you did?
is the drop in mpg so significant it will prevent you from driving your truck at certain times?
whats the big picture and what does it really mean?

 

I get my best mileage in the 70-77 mph range. I never kick it in and start out steady as she goes and gradually reach that speed.
I'm also in Arizona - nice flat highways and never drive during commute hours, one of the blessings of being retired I skip all that
slow and go traffic.

 

15 miles is not a test

 

New tires, you say?...