@Vman what do you have for a jeep? Is the 3.8l auto good for off an on road with 33's. Curious if it will rev high on highway
I have an 07 3.8 auto...so same as you basically. Mine has a dana 30 front diff which I think all the "x's" have, and is a 4 door unlimited.
There's a few things working against you with an 07-11: 4 speed automatic transmission, the 3.21 or 3.73 gearing in the diffs, a little less horsepower from the 3.8 compared to the 3.6, and for the non-Rubicons the dana 30 diff in the front.
Once you start throwing on larger (heavier) tires, the gearing in the diff and the shift points for the 4 speed get really bad. Even with 35's my JK would struggle going up a grade/hill and the transmission was shifting all over the place. I'd have to turn off overdrive to stop it from going bonkers going up a hill. Putting 37's on a dana 30 is a lot of weight that could break your axle just hitting a pothole.
Here is a great link to compare the size of tire you want to run, with either auto or manual, and if you're running a 3.8 (07-11) or a 3.6 (12+) JK. Blue would be a trail only rig, green is fine for highway driving, and yellow is underpowered and bad for both. It lists what your RPM's would be at 70 mph I think.
ROPER GEAR RATIO TIRE SIZE: JEEP GEAR RATIO A frequently asked question we hear is "What is the proper gear ratio for my tire size?" When you change tires on your Jeep from the stock size to the larger tires to give your lifted Jeep the look you want, a change in gear ratio is necessary to give...
prodigyperformance.com
I can say that my '07 unlimited x (sport) on 35's with 5.13 gearing runs great. Transmission shifts naturally, you don't get the loss of power feeling, and the RPM's at all ranges are not a problem.