I have been trying to find something about a jku with 3.92 and have not found anything. I'm sure there is a reason why no one goes that route. I a '12 jku auto with 3.21's and the 29" tire. I will be putting on a 2.5" with 35's.

From the gear chart, and using online RPM calculators, I think the only thing I have to worry about is highway. It looks like 3.92 will put me at the same RPM range with the 3.92 and 35's as I have now with 3.21 and 35's. I say I think take off will be ok because I will be at the same around the same 250 ftlb at 2k RPM.

Is the measly (roughly) 100 hp at 2000-2200RPM enough to keep that meat turning on the highway without shifting too much?? I live in DFW so its pretty flat, and I don't crawl. Mainly use for overland and backroads to the deer lease.

If any of my theory is off please don't hesitate to correct me. I don't mind learning new things.

 

I have not seen 3.92's for the JK's

 

I have never seen a 3.92 offered on the jk dana 44 only 3.21 3.73 4.1 4.56 5.13 and 5.38
That may be the reason you are having a hard time finding it.

 

Your theory is correct, just the ratio is wrong.

Plug the 'measured' height of the stock tires with the stock 3.21 gears into the chart to find the stock rpm. Then substitute the 35's (using actual measured height again, not mfg spec), and find the gear ratio that puts the rpm closest to where you want to be.

Sounds like that will be somewhere in the middle of 3.73 and 4.10 for you, so base the decision of slightly higher or lower rpm on your actual needs.

 

I used the 2166 for the 3.21 with 29 from the gear chart and the 3.92, 35, and 0.83 for the OD in the calculator. I only used 70mph, not lower to see what the rpm were at.

 

That would make sense.

 

Keep in mind that it is not as simple as looking at the RPM on the gear chart. With 35's on a JKU, you will spend a lot more time not in overdrive if geared wrong. I have a super light 2013 2-door with 35" Duratracs and 3.73 gears. It is just barely tolerable on the highway. To put it another way, I would get better MPG if I had 4.10's. I shift a lot on the highway and unless it is flat with no wind, I am never in 6th gear. Lugging the engine is not the way to go. You should be deciding between 4.10 and 4.56. My opinion anyway.

 

Great point, which is why I posed the questioned. I have had that exact debate going in in my head since I bought it a year ago. Deep pending in the outcome here, I'll go with the 4.10's. Probably not going to 4.56. I can't justify that low with 35's for what I do. If I had the coin I would go 4.56 and 37's.

 

4.10's will be a good result. The only downside is that they are hard to find and twice the price. Check the vendor section, one of them was advertising 4.10's.

 

I don't know why everyone says this. I've gotten several quotes for 4.10 and 4.56 and everyone has said "they are the same" lol

OP, don't think 4.10 would be too much gear at all. 2500 at 70 is just about perfect.