So i got a 2012 Sport 2dr JK automatic with the 3.21 gears and i just installed a 2.5 teraflex kit with bilstein shocks,which is amazing ride quality is superior than stock! anyways i really want to run 35s nitto trail grapplers cause i love the look of them but knowing i got the 3.21 gears i would hate to loose all my efficiency in gas and become a gas guzzler, what do you guys recomended? and how bad would my gas be if i stick with the 3.21

 

I can't speak for auto, but my 2012 six speed with 3.21's and 35" km2's has been seeing 18 mpg keeping it under 65 mph, all highway. I dd it 60 miles a day for work. I'd expect 14-15 or less mpg average depending on driving, considering yours will be a lot heavier. You will want to keep overdrive off as well. Ideal? No. Doable, yes. Worth it if you don't use it offroad, probably not depending if it's your dd. I have no regrets. I'll regear to 4.10 or 4.56 eventually. Difference offroad makes it 100% worth it

 

Unfortunately my jeep came with 3.21s as well but I installed a RK 2.5 with fox shocks and I'm running 35's in an auto transmission. Like said by kh202 it's doable, I get roughly 13-14.7 mpg city/highway but re gearing is in the immediate future. I currently live in PA where there are plenty of hills as well.