[theunchosen]

I think there is a mosconception about unsprung weight.

Unsprung weight typically causes issues with wear and tear because any impact the tire takes is translated to those unsprung components as well.

You also see a large reduction in MPG if you reduce the unsprung weight. However, in most cars the only real unsprung pieces that change weight are the tires. That said If you ripped everything off thats not sprung you only get the efficiency of less weight. If you shrink the tires you gain in two directions. You gain because you are not having to move that weight forward and also you gain because you are not having to overcome as much force to make the tire rotate at speed.

More mass further from center means more energy required to make it move. Tire wind resistance is small compared to the amount of energy it takes to overcome uniform resistance. The center of your wheel most likely only makes up 1/5 of total wheel weight where the rotation is free and 4/5 are allocated along the edges where its most expensive. The tire is entirely on the expensive side.

The further from the point of force the mass is the harder it is to turn it. Push a door open from the hinge instead of the handle. It's like that for your tire. If you can make the door narrower(less distance from hinge to handle) it becomes easier, or if you reduce the mass of the door. It's also the reason racing components are very lightweight especially any component that spins along the drivetrain(clutch).

You could replace standard wheels and tires for much lighter models of equivalent size and that would allow for no alterations to speedo or rpm/speed ratios and that would increase MPG as well.

If you really want to go big I'd recommend small diameter tires and wheels(standard profiles for wheels mud tires drastically reduce FE because of rolling resistance and extra energy needed to rotate them.)