The graph posted above is for the 2007-2011 3.8.
Here is one for the 2012-2016 JK with the 3.6.
Big difference between the two.
Been research this myself. Use to have a Rubicon manual. With 4.1 gears. Once upgraded to 315s, ran fine. Re programmed as well..
Recently test drove a used 2015 JKU with 3.73s and 35 x 12.5 MTs on it. Drove like a dog, just did not have any get up and go torque or power.
I am in the same boat. Was going to get a Willy's that already has the 3.73 gears, or order a sport and do the $695 up grade to 3.21. But, since looking at putting on 35s/315s (which are actually about 34.5"), be better to put that money towards a re-gear which is going to run around $1,100-$1,200.
Seems to me, from research I am doing, is that 4.56 gears is the sweet spot for 315s or 35" tires. If you have Auto transmission? (Going Auto this time, was not happy with the JK's manual last time). 4.88s if you want to go 37s.