First ones easy, bought the truck 3 months ago, first check on mileage was about 18. Did a complete service, now getting high 15 to 17

I've been driving with a light foot till I feel out the truck's bugs, not many highway trips mostly back/forth to work. What is the problem ive heard of 20-22 from some guys.
Next, tranny seems to downshift sometimes just before overdrive.(like its searching for the right gear) Doesnt slip and doesnt do it very often. Its something that just happens and i cant "make" it happen to try and get it diagnosed.