Re: 20 inch rims and mpg?
Big wheels and tires are for looks, and are costly. Better off upgrading the worn stock suspension, than putting on 20" wheels and tires. Mine with 17's and the Koni suspension will easily outhandle a stock WK with big wheels and tires, plus rides much better, is great off road, and is faster. If you were to put on the same tires in 17 and then 20, the handling difference will honestly not be that much different, but the ride will be quite a lot rougher over bumps, and forget offroading in your Jeep.
2006 GC Overland QD II
5.7 Hemi, Modern Muscle 85 mm CNC ported TB, Apexi super voltage stabilizer, Billet Tech. Catch Can, Koni FSD shocks/struts, SRT8 sway bars, Cooper AT3's, Amsoil fluids, Diablosport Intune CMR tuned by Hemifever tuning.