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I think I'll join the bad mood band wagon just cause
Anyhoo, you are saying you are slapping on tire and you are changing the gearing so that it spins at 3k @ 55. Kewl. You do this for each tire. Congrats, you have just equalized the tires in regards to size to the power band. Everyone running a lifted truck does it all the time. You have negated the tire size.
Now width. If all of the tires have the same width, they'll get the same MPG because as far as the engine is concerned its spinning the same tire. The wider you go, the less MPG you get. In a perfect world, we'd all drive on mag rails
Now rubber composition. If each tire is made out of the same stuff, same MPG. If one is a LRR tire and the other isn't, the LRR tire wins in the MPG
Kewl
Now rims. Assuming the same alloy of materials and density, a smaller rim wins. Reason for, a bigger rim needs more mass to be bigger. Hence forth its heavier, and harder to spin, makes the engine work harder, drops your FE. As an experiment, pick up a steel 13" rim, then pick up a steel 16" rim. Noticeable difference.
Now Tire diameter, no idea why you'd want to run a 5" rim, you'd have brake problems and stuff. Bad mojo, but your call. The only benefit you'd have is that the tire would be super narrow compared to say a 14" rim and tire, and much less weight so its easier to turn. Depending on the aspect ratio of the tire, you'd still get sidewall flex. But if you are running at max load, you'll negate a fair amount of it.
Forgive me, I was bored.....
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