This is discussed to death in my Jeep forums.
Most of us with Lifted Wrangler JK unlimited get about 12-14 MPG.
The benefits of Premium vs regular is nil.
Premium vs. Regular | Car Talk
Octane is defined as a fuel's resistance to knocking. There is no benefit if the octane is higher than what the engine needs. Engine knock occurs when fuel in a combustion chamber ignites before it should. This disrupts the engine's operation. But electronic knock sensors are now common and have nearly eliminated engine disruption.