[zonker]

in theory, assuming all other factors are equal (which they rarely are), the best selection is the smallest rim diameter with the tallest tire.

rim diameter: the heaviest sections of a wheel are at the rim, and the heaviest section of a tire is at the tread, so lower profile tires and larger diameter wheels, given the same tire outside diameter and total weight, will require more energy to rotate.

so in the event of adding a larger diameter tire than what you have:
if there is a way to move the heaviest part of the rim further from the tire OD (smaller rim diameter) it would help offset some of the flywheel effect you will get from a larger diameter tire.

from my own personal and very recent experience, my 2.3 mustang was originally shodded with 195/75-14 tires and polycast wheels (steel rim with a alloy looking plastic molded over the steel). that combination weighed 36 lbs. In a pinch, I had to replace the tires with 215/70-14s on the same rim, tire diameter increased under a half inch, but tread width grew and weight jumped 2 lbs. my previous best mpg was 41, but now my best is 36. I'm not saying the 5mpg drop was solely tires, but my guess tells me they did contribute.

my mpg tire/wheel plans have me looking at air cooled beetle tires (165-80-15) mounted on factory mustang cobra alloy spare rims (15x4, 4x108, 28mm offset, 8 lbs each). i should shed over 10 lbs a corner, reduce my contact patch and still maintain the stock tire diameter. I'm sure my tradeoff will be marginal stopping distances and tire adhesion, but I'm willing to accept that in pursuit of eco efficiency since i do not drive this car at 10/10.