It's a little more complicated than just going with larger diameter tires...

1) Rolling resistance changes between different tire types/sizes. This will affect fuel economy a small amount. Could go either way.

2) A taller tire will raise ride height, which will probably increase aerodynamic drag slightly. This will also hurt fuel economy a small amount.

There are online tire size calculators out there that show how changes will affect engine RPM.

And don't forget the larger dia. tires will also cause the odometer/speedometer to under-report distance/speed vs. your current tire. If you don't compensate for that change, your apparent fuel economy will suffer.

FYI, I changed the final drive gearing in my car to reduce cruising RPM by 25%, which is massive (much more than you'll get by changing tire size). My steady speed fuel economy on level ground improved by about 5%: Project 'nerd gear': taller tranny transplant nets +5.2% MPG - MetroMPG.com

Not saying it's not worth doing, but the effect will be fairly small, and you'll want to research tire characteristics and the potential payback before spending the dough.