I can get 20mpg if I keep somebody in front of me to reduce wind resistance and stay around 65mph. I saw 23mpg once but that was doing 50-55mph and babying it. A Rubi won't get the advertised mileage, since that's for the base Sport model with the 3.21 highway gears and street tires. Once you factor in the mud tires (usually good for at least 1mpg drop on their own) and either 3.73 or 4.10 gears then it'll drop more, especially if you're keeping the speed at 70mph or higher.
I'm more annoyed that my city mpg numbers are only 12-13mpg. I drive mostly "suburban" with only an 8 mile round trip work commute. In the 3359 miles recorded since I bought it, the running average is 13.5 with the best tank mpg of only 16.1. And this is mostly on flat ground, you have to look hard to find a hill in my state. I drive it fairly conservatively as well. My numbers estimate is that only 30% of my driving has been on highway, so that will affect it, but I wish I could figure out how to get the city mileage higher.
From looking through the stats of other 2012 JKs on Fuelly.com, the only ones that get near 20mpg are either Sports or do primarily highway driving. https://www.fuelly.com/car/jeep/wrangler/2012
2012 Silver Rubi. AEV 2.5" lift, JCR Dagger front bumper, Superwinch EPI 9, JCR engine skid, ACE side steps, RR 7" HID lights, Wired HDH switch panel
formerly- 2001 Yellow XJ, 7" longarm, lockers, bumper, winch, armor, lights, etc