What did you mean by "economical"? Did you mean like saving battery energy or to purchase the bike itself?
Anyways, fat tires are absolutely terrible idea if you're looking for efficiency.
Although I'm not an engineer, being a motorcycle enthusiast, I've read a lot of articles written by motorcycle engineers.
If you look up for words like "rotating mass", "rotating inertia" and "unsprung weight" you will quickly find out how bad it is to add weight on wheels.
To put it simply, putting a weight on "moving component", especially rim/tire is whole a lot worse than adding weight on frame.
I know a lot of bicycle companies love to talk about light frame, such as titanium and carbon fiber, however the saving weight off frame doesn't do much.
The overall motorcycle weight (in this case, bicycle) doesn't give you the whole characteristics.
I'll give you an example of "moving component": Think about carrying a 10 lb backpack, will you be able to walk or run normally? The chances are, you can. Because your back isn't a "moving component" when you're walking.
Now, let's put that 10 lb weight somewhere else.. can you walk or run normally when you wear 10 lb shoes? No you cant.
Your overall weight didn't change, but simply moving weight from frame(back) to wheel(feet) has drastic difference.
This is exactly why high performance motorcycle engineers are trying to shave the weight off any moving components.
No wonder I could beat 750W fat bike with mere 350W commuter bike.
On top of added rotating mass, fat tires drastically increase friction too.. it really doesn't make sense unless you're riding on snow or sand.
