I have put 33's on my 2012 auto JKU. According to my window sticker I have 3:21, but at 70 mph I am running 2150 rpm. Now I am starting to wonder if that's what I have or not.

 

So I hate to be "that guy" who asks about best MPG for a Jeep, but...

Are people seeing better MPG gains with lower ratios (such as 4.56 vs. 3.73 on 35" tires) or is the higher RPM range hurting MPG? It seemed like my older JKU with the 3.8 litre got better mileage with 5.13s than it did with 3.73s for 35" tires, but I attributed that to lack of TQ and HP. So I guess my question is this; Since the 2012 has more HP and TQ, is it best to keep this motor in the lower RPMs, or is it best to get a little higher in the RPM range and make it easier on the motor to yield better MPG?

 

I would not see a need to go above 4.56s in a 2012.

I'm no expert on this as this is my 1st Jeep, but 4.10s to 4.56s seem to be perfect for 35s-37s.

I have 3.21s running factory tires and I just don't see how making a small bump to 4.10s with bigger tires will make that much difference in MPG. I think the drag created by a wider tire and the tread pattern (rolling resistance) would be more of a factor than gearing higher while adding circumference to the tire (almost a wash?).

 

Auto Comparo~
I put about 60 miles a day on my 2012 JKU(3.73/stock tires). Driving rolling curvy back roads, using the "Eco On" as much as possible I can stay over 20mpg. The 3.6L lugs up hills without downshifting and having an extra gear and torque keeps it in the sweet spot better. With the 4 speed auto it has to step further out of the sweet spot to maintain speed. Regearing the axles based on these charts keeps you in the optimal MPG range for the 3.8L Auto. Higher RPM under less load can yield better MPG than lower RPMs under load. A vacuum gauge will tell you the same story.

 

wth is "Eco On" ?

Am I missing a button in my 2012?

 

As near as I can tell it's not something up you can toggle on and off. It just tells you whether your driving economically or not.

 

It's telling you you are driving the most economical way at the moment...you can turn it off in the personal settings section in the dash

 

Sounds about right, exactly what I'm turning.

 

Note, it will only allow you to access personal settings when you are in park.

 

what was your mpg at before ?

im going to be doing 35's soon on the same (4:10's on a 2012). ill try an get an accurate before and after, fwiw.