With my Grand Cherokee (same straight 6 as the small Cherokee and Wrangler), I get about 17-18 city and 20-21 hwy. Not that I do any "city" driving any more
19-20 average for "mixed" driving. I did about the same on my small Cherokee. I think the Wrangler suffers from its non-aerodynamic body---lots more places for air resistance than a Grand Cherokee.
If you go with the Wrangler, don't try to save an mpg or two by getting the 2.5 liter 4 cylinder. The difference between that and the straight 6 is night and day.