I was surprised that after all these years, they haven't really improved the mpg of the JK over the TJ. Several guys who own both a TJ and a JK (post 2013) told me, their TJ does as good or better in mpg. Of course it depends which model I suppose.
The lure of the Pentastar's 275 HP engine had me excited. I did test drive a few. on the JKU, on the two door it was nice, but not as much power as I thought. I guess the added weight of the JK counters it a bit of the power increase and mpg too.
I still believe tires really make a big diff in mileage. Since getting my dreadful (according to some here! lol) E load K2O's on the day before Thanksgiving, I've had to fill up only twice for my DD! On the stock tires I would have filled up 4 times by now. Immediately getting those tires put on, I drove 6' out of the garage and noticed the jeep was rolling almost effortlessly. Thinking back, I'm so glad I didn't trade for a JK, and decided to put a few grand into mine.