Well, your aspect ratio is staying the same. That's going to change your final drive ratio.
A narrower tire will reduce your contact patch, which might increase FE, with the same aspect ratio, your speedo will read differently in addition to giving you different readings for your miles covered.
Yeah, you'll end up with a shorter tire and more RPM on the highway like that so overall I'd think it would hurt FE. You probably need a 175/70 or something.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice