That's a good question. I would think the answer would be that it's no different in that case but otherwise you get the increased torque in all the other gears.

Anyone else gonna take a crack at this?

 

Your math is right on and there would not be a difference between driving in third or OD in the case that you provided, but the difference is going to be that you lose a gear. Also, your take off will suffer and if you try to down shift at high way speeds you will be down shifting to 2nd and the jump between 2nd and 3rd is not as smooth going down as it is up. It will also put way more stress on your drive lines, U joints, T-case, axles and bearings. You will also wear your gears out to the point were they will break. I have seen a teeth ripped off the ring before not fun to fix, fun to watch how it happened but not fun to help fix.

The last thing to look at is that it is a ratio. Your transmission and axle final ratio is only 2/3rds of the problem. Stock tire size compared to new tire size. If you like the way that you jeep drives stock then it is simple use 4.10's and you are good for 35" tires, but if you feel like most do that our jeeps are still under powered (note to jeep 6.1L hemi or cummins is a must) then a little bit more gear will allow the jeep to act and handle like it is supposed to.

 

Thanks for the response and those are great points. By the way, I am all for regearing and I know that is a matter of preference. I am will be putting in 4.88s this weekend. I am opting for the rubicrawler to get the better crawl ratios. My Jeep is my daily driver and with 35s, I have OD off most of the time. In most of the forums, it seems that the preferred gears are 5.13 or 5.38. From a crawl perspective it makes sense. I guess I don't see great gas mileage in my jeep in 3rd gear and I don't see how going to 5.38's would improve highway gas mileage.
Rob

 

The most important thing is keeping your cruise rpms at highway speeds between 23-2700 rpm. That's where you get the most efficiency, with nothing more than a nominal difference in fuel mileage between the two. The more hills you encounter, the more being closer to 2700 rpm will pay off. If your busting it on the flatlands, 2300 rpm will be better. Then of course, opting for a steeper gear set is going to help off road.

You can beat the decision up a long time and the verdict is always the same. You aren't going to hurt the motor by spinning it more, and there is efficiency to be found at moderately higher rpms. As always, the biggest issue with the auto transmissions is the amount of kickdowns, and what you can reasonably do to avoid them is always the best bet.

 

Well after looking at this as the mother of all upgrades, And thanks to my own old school experience, And everyone's input here and on the forum... Thanks again, great input EVERYONE...
I also own a different Jk now since this thread was started, a Rubicon Unlimited and I have finally decided to get 5.13's... fwiw, I'm running 35's now.
I get alot of travel time on the freeway, so hence the 5.13's instead of 5.38's...
These gears are "fully" acceptable on the freeways, and produce "less heat"
at the pumpkins (5.13's vs 5.38's) when sustained speeds are at or above 70mph.
At this point.. This is where "my concern" is, Not the rpm's of the 3.8L and I can see why some D44 people choose the 5.38's as well.
For me and my JK driving needs, 5.13's with more gear contact made more sense..
I'll update this thread when I get them installed too.

It would be interesting to see how "long term use" of 5.13's and 5.38's do on the freeways.. time will tell... I think Jpop's chart is great too. It enlightens people here when compared to the manual guys revving the 3.8L and getting decent mileage.

 

I'm curious as well

IMHO, everybodies needs\wants\desires are different, what works for you *may* not be the best choice for me. Example, my Rubi JKU auto is not a DD and I don't drive the interstate (freeway) that often, so I felt 5.38's were the choice for me even with KM2 35's.

Either way, the gear change should be better than stock

 

My point as well... fwiw, If my driving needs were like yours? I'd have 5.38's too! Great choice.

 

Good point in regards to the heat buildup risk with using 5.38s, which has not been mentioned much. The shop that is going to install my gears has warned me about the potential heat problem that 5.38 can produce. Is this really a big issue?

 

In my opinion it's new territory here, In my previous experience being around people who have D44's with 5.13's.. They do produce some heat for sure... Especially on the freeway which in the past you wouldn't hear some one running D44's with 5.13's going down the freeway @ 70mph. That is some pretty serious driveshaft RPM's...
Remember our .69 OD is "not" slowing down anything after the transmission.
With that said, I know one person who has ran 5.13's in a late model 96'Bronco with 37' MTR's and has ran this setup for a number of years. Yes, he has used it on the freeway as well, his concern was addressed by keeping his speed down below 65mph which he felt is a comfortable spot... But he now is looking for bigger axles, but no breakdowns as of yet.. I hope this helps somewhat..
Maybe starting a new thread might begin insight on this concern moving forward.

 

There have been some posts talking about high axle temps during break-in, but I don't recall any threads concerning gear problems due to heat. People have been running 5.13's in these axles since 07, so if it was an issue, I'm guessing we would be hearing about it by now.

You could always run 140w in both axles?