I have 2 gearing related questions for the JK braintrust.
1) 3.73 or 4.10
2) Detroit Lockers, Selectable lockers, lunchbox(?), or just leave it alone with the factory trac-loc LSD in the rear?

The Background:
This is my daily driver. 98% of my miles are spent creeping down the highway in Houston rush hour traffic. I’m lucky to average 30 mph. 1.5% is open highway driving at 60-70 mph. Every now and then, I’ll take the family out on the beach fishing or up a mild trail for camping. That accounts for the other 0.5%. Even in Houston traffic, I still get pretty good mileage (19-21 mpg), because I’m a real laid-back casual driver.

The Build:
I’m starting with a 2012 JKU Sport with auto and 3.21 gears with Dana 30 in the front and Dana 44 in the rear and Trac-loc Limited Slip Differential. It currently sits on Goodyear P255/75/17 tires. I have a 3.5” AEV DS SC lift on order, along with ACE rock sliders and rear bumper and I’m about to pull the trigger on a set of 315/70/17 Duratracs. As I said, this is my DD, but several sets of tires down the road, a set of 37s aren’t out of the question, especially as the kids get older and we become more active. This brings me to the regearing questions. I have a pretty decent budget (+/- $500 won’t make a difference to me), but I don’t want to throw it away on something that I won’t ever use.

1) Should I regear to 3.73 or 4.10? I think I’m one of the few people who like the 3.21 gearing, maybe because I’ve never had the 3.73 or 4.10. When I say “like” what I mean is I like the mpg and tolerate the lack of acceleration. So, when I do the math: (3.73/3.21) * (32”/35”) = 1.062. If I go with the 3.73, I’ll be at 6.2% higher rpms for the same speed after the tire change. The same math for the 4.10 works out to 16.8% higher rpms. If I’m okay with the current acceleration, the 3.73 will only get better, even after the tire change. Why would I go all the way to 4.10 and take a bigger hit on mpgs? I can only think of 2 reasons: First is the potential future jump to 37” tires and second is that maybe my mpg would actually improve (based on my driving habits and use) if I had the 3.73 today and I just don’t realize it?
2) Lockers? This is a subject in which I’m fairly ignorant. A mechanic I was getting a regear estimate from asked me what I wanted to do and I couldn’t even answer the question. As long as this is my DD, my offroading will be limited to driving loose beach sand and a fairly mild fire road trail to a camping site. However, I NEVER want to get into trouble; if I only use the full capability of the “device” once in the next 5 years, I will consider it paying for itself. On the other hand, I don’t want something that scrubs power (mpg) or accelerates tire wear during the 98% of miles described above and it must be dependable. Am I asking too much? What am I looking for? I watched some Youtube video reviews of the Trac-loc LSD, and wasn’t too impressed with how it got hung up on some unfortunately placed bumps. And then there’s a question of front or rear?

Looking forward to reading your advice. Thanks.

 

Most definitely don't waste your money regearing to 3.73. You want 4.10 or 4.56 with that setup. Here are the charts. From the way it sounds, I wouldn't bother getting a locker if you only do moderate stuff. no locker will get you "never in trouble" it's all about the driver. My open open jk keeps up with locked rigs a lot. Do lockers help in some situations sure. But you don't need them for forest roads :p

 

Some great locker info from the folks at Dynatrac:

http://project-jk.com/jeep-jk-tech/d...ts-lockers-101

 

I have a 08 JKU 6 speed with 3.21 and the LSD in the rear and this weekend I was out at Hidden Falls for a big wheelers for the wounded event and was hanging with some if the big boys really well. I was thoroughly impressed with the way the LSD worked and I am in a regear/reaxle fight with myself right now! I want to regear to get some of the power back that was lost when I put 33's on, but having the 3.21 means I need a new carrier up front for the 4.88s I want to run. Once I add that into the equation I am starting to see a d44 being a better overall option. Mine is a dd as well and most of my miles are spent under 60mph with the odd road trip here and there. My rig currently is pretty worthless on the freeway in 6th gear, I have to pretend it doesn't even exist most of the time because I just can't stay in that gear. I'm running in 5th and sometimes 4th just to stay with traffic and passing is not an option.

 

A front carrier is only like $75-80 bud. Not that big of a deal in relation to what you're spending to regear

 

I have a 2010 Rubi with 4:10's and I run 35"s. I also have the 6 speed not an auto which makes a bit of a difference.
My experience with now having 40k miles and running 35's - if you are changing gears with a plan to run 35"s, I would go more than 4:10. I would say that a set of 4:10 would give you better than you have now but a bit lower like the 4:56 or 4:88 would be better. The 4:56 with all highway driving - like myself, would probably be a sweet spot.
With the 6 speed I find I can run with 6th gear as long as I keep the RPM in the 2500 range or above. With the 35's and 4:10 I am just under at 55 - 60.
I believe the 4:56 or 4:88 would tweak that up just enough.
As to gearing now compared to 37's in the future - you might want to gear and put the 37's in now. Just a thought.
Enjoy.
Stay safe
Mandor6863

 

I'm kind of in the same boat as you. By the end of the week I'll have 35"s on a 3" lift with 3.21 gears.

I am concerned with the performance with the big tires and high gears, but I want to shake it out first.

My advice would be to wait and see because you don't want to be soaking too much money into a D30 if its not going to be the right tool for the job.

The issue is the small R&P, and it gets more vulnerable with lower gears (high per numbers).

My assessment would be that if you find yourself in a situation where lockers are necessary than you'll probably want to be on something bigger than a D30.

From my previous OR experiences there are very few situations where lockers are "necessary".

With that being said if you are going to pay someone to change your gears you may as well get lockers at the same time.

 

Can you say a few more words about the 3.73 being a waste of money? Do you say that just because of my potential future plans to add 37s? I like those charts very much. I built an excel spreadsheet that reproduces that chart for every gear (transmission, not diff) and for any speed (if anybody wants a copy, just let me know). As I interpret that chart, going from 3.21 w/32s to 3.73 w/35s will be a net reduction in gear ratio, yielding higher rpms for a given speed. So if I’m okay with the stock performance, a 3.73 will be better (by 6.2%). Sure, 4.10 would be even better (16.8% higher rpms for a given speed), but my assumption (right or wrong) is that improved acceleration will come at a price of lower mpgs. I know I’m simplifying things by neglecting the effects of the additional rotating mass at a larger radius. Including these affects involves more physics than I want to get into, call me lazy.

I also reproduced the 3.6L power & torque curves (I’ll try to attach below, hope it works). This curve tells me that I’ll get no more torque at 3000 rpm than I would have at 1800. According to the gear chart, at 70mph with the 3.73 and 35s, I’ll be 2081 rpms and the engine will be producing 250 ft-lb of torque. If I went with the 4.1, I would be at 2288 rpms, but still only be at 250 ft-lb of torque. So if the engine is producing the same torque, you should have no better ability to hold your speed against wind or up a hill. I’m hoping someone can straighten out my argument here. I must be missing something.

 

I'm not saying that 3.73's won't work, they will be better than 3.21 for sure (obviously). I'd just hate to spend all that money and be in the same boat of people with stock 3.73 saying "yeah 3.73 is OK but certainly not ideal. From what i've heard from first hand experiences, 4.10 and 35's is pretty much perfect for these new engines all around, 4.56 might be a little better for off road performace. I still have 3.21's so i can't speak as to why 3.73 wouldn't be ideal, but all i know is there are a lot of people with stock 3.73 who aren't happy with them. Just my 2 cents

As for the torque curves, i don't really follow. You most definitely have more torque at 3000 than you do at 1800. I'm at work and didn't open the attachment, but for example, try holding a certain speed in 6th gear on flat highway or try to accelerate at 1800 rpms. Now down shift hold that speed and stomp on the gas.... make sense? The 3.6 and the 3.8 are definitely high revving engines compared to the 4.0

 

I think the 2010 manual and 2012 auto may be somewhat of an apples & oranges comparison. But Dave at AEV suggests when comparing gear ratios between the two you should go one gear higher for the 2012 engine and trany. So, with that conversion in mind, I think you're building a good arguement for the 4.10.