Most likely the wheel weight man.
I went from 15" stockers to 17" 15 spoke stockers and 17" motegi 5 spokes I lost a handful of mpg I could never regain at least not very easily.
I never weighed them to compare but it was noticeably heavier when I was picking up the wheels to bolt on. I lost about 5-9mpg by going to 17s but oh well.. you gotta pay to play. lol
However, Since my Ti I just got is starting with 18s I wonder what I'll gain from going to 15s in the winter.
1965 Mustang Coupe 289 4spd
2003 Land Rover Discovery II SE
2013 Lincoln MKZ Hybrid