My 2010 Rubicon Unlimited has the 3.8 and 6-speed manual with OEM 4.10 gears. It's on 315/70R17 Treadwright Guard Dog M/T's and a 3.5" AEV suspension. I have used a programmer to dial in my tires' exact rolling diameter, with
correct mpg averages reported with a ScanGauge II (much more accurate than the fudged readings from the OEM mpg display).
Daily driving (mostly city, with a few short jaunts onto the highway for a few miles) nets tank-averages of 13.5 to 14.5 mpg.
A recent 1,100 mile trip to (on flat highway), through (roughly 300 miles on dirt), and from (windy mountain roads) Death Valley with the hardtop installed averaged 19.2 mpg door-to-door. Yes, I was surprised at that number too. I double-checked my numbers and recalculated manually, confirming it's correct. Perhaps the tank of gas I got in Nevada was better fuel than the ethanol-diluted California fuel I'm usually stuck with, but I usually have no problem maintaining 17+ mpg at 70 mph.
Another recent 500-mile trip (on flat highway) while towing my Lifetime Tent Trailer (again with the Jeep's hardtop installed) returned 16.1 mpg from mostly 60 mph cruising.