bigger wheels = less fuel economy?

Depends on the weight and also on if you correctly plus-size the tires or not.

If you dont change the overall diameter of the tire/wheel combo, which you shoudnt unless you also change the speedo gearing, and the weight of the wheel/tire combos are the same, you should see no noticeable difference.

shoudnt unless you also change the speedo gearing, and the weight of the wheel/tire combos are the same, you should see no noticeable difference.

Giggle Cream - it makes dessert *funny*!

If you keep the tire outer diameter the same, and wheel diameter goes up, and everything else (width, mass, inertia of the tire/wheel assembly) remains the same, you might make a case that your MPG would increase very slightly, albeit immeasurably so. That change would result in the sidewall height decreasing, resulting in less sidewall flex. And less flex is less energy dissipated in the tire, i.e. less power wasted. Thats my guess anyway.

That's really a stretch here bringing tire flex into it. Look, your mileage will essentially be the same if the outside diameter of the tire doesn't change. Even if the diameter does change a little the mileage might appear to change based on calculating it from your odometer but if your tire (outside diameter again) is a different size than what came on the car you won't be measuring actual miles - maybe more with smaller tires (outside diameter) or less with larger tires (outside diameter). If your new tires are wider there will be a greater contact area and more rolling resistance which might reduce mileage a timy bit but this along with the weight of the new wheels and tires compared to the original weights will only make a tiny and probably imperceptible difference.

If you want to get into some big abstract theoritical quantum physics analysis like how much the railroad track flexes when the fly lands on it or how much the ocean level rises when you piss in it that's fine, but the bottom line here is pay attention to the outside diameter of your tires before making a change.

I like your analysis.

Although if you choose tires with higher rolling resistance, you may lose fuel economy from that. E.g. if your OEM wheels have something like OEM Michelin Energy MXV4+ tires, you might find that most other replacement tires (on OEM wheels or other wheels) have higher rolling resistance and lower fuel economy.

That damn fly on the railroad track question is gonna bug me all night.