I do not think the induced air drag due to the change of wheel width is in anywhere significant, although an increase in wheel height (which makes car taller) should make some difference overall - and probably only their engineers would know whether allowing more air through the floor would make a positive or negative impact.

And bigger tyres should be beneficial for cruising because it can carry more energy on itself thereby taking longer time to lose its 'charge'. The only major setback is the reluctance to accelerate from rest but once you get the wheel moving it should be alright.

Of course, we should be aware that suspension setup etc. will make an effect and that could affect FC as well.

Anyway, let's do some calculation - given there's an inch gain in diameter when you opt for an inch bigger (just assume the side profile is same for old and new rim size). To travel 100km, old tyre (say 15" rim with 185/55) will need to revolve about 54,000 times while new tyre (with an inch gain in diameter) will revolve about 52,000 times, a 4-5% gain in lesser revolution. If just pure cruising (which eliminates the more reluctant to start from rest deficiency), this gain should be pretty accurate. Whether the other factors will offset this gain will need to be carefully experimented only can know - and will also differ between cars for even more factors. LOL!