Is there that much difference between the 1.8 and 1.6 engines

?
I got my 1.6 Sedan with 5MT new in March (7 months) and at 17 000 miles it has averaged 42MPG. That is calculated from the log where I write down every fill-up. That is driving hard on the back roads in the mountains, revving past 4k rpm often, using the A/C, etc. But that is also with synthetic oil in the engine and transmission and tire pressure at 36psi.
Do I just get better fuel locally? I worked in Michigan a few years ago and noticed that my Frontier went from 28MPG close to home to ~21MPG in Michigan, and back to 28MPG when I got home, probably due to different fuel.
Maybe it's just me.
Ben N1NP