Josephtate

Hello all,

I have a 2011 JKU sport, I’m fixing to put 315/70R17 Mickey Thompson Baja MTZ tires on it. It’s an auto and I live in the hills of east TN so gears are an absolute must for me, I also do a lot of wheeling in Windrock and RB off-road parks as well as other areas. Lots of rocks and sticky sticky mud. I’m bouncing back and forth between 4.88s or 5.13s. I’m a mechanic so I know how to install and all that my concern is top end speeds in comparison to RPMs. It’s a Jeep I’m not looking to drag race but it’s my DD and I do hit the interstates quite frequently. It’ll be an approximated 35” tire so the little charts online say 5.13 optimally but 4.88s will work. I don’t drive fast I’m no lead foot but speed is a slight concern as speeds here on the interstate are 65 while everyone goes 80 lol. I know the 3.8L/auto combo is sluggish and all that so I want some more pep on and off road. It’ll have a steel stubby bumper with a 12k winch soon, rock sliders after that, and eventually a steel bumper and upgraded hinges for a 35” tire as well as some other modifications. So weight is gonna absolutely climb.

If anyone has any input it would be greatly appreciated thanks!

tjkamp

Not sure what to say on which way to go, but I do know all of the gear charts are for measured tire high with the weight of the vehicle, so for a 315 (35) you should be looking at the numbers for a 34. When I adjusted for tire size with the tuner when I went up to 315s, the factory setting for a rubicon on 32s was 30in.

nthinuf

Performance for a 3.8 auto on 35's, 5.13 would be my choice.
Rocks and sticky mud, some folks will start thinking more about the strength aspect of that ratio in a front D30.

Guess it comes down to your opinion, and how you are likely to drive: (strength vs usage). Does testosterone/ego/whatever have you powering through everything, or are you more likely to stop and break out the winch. I guess the way I see it is that if 5.13's were to implode, what are the realistic odds that 4.88's would have survived? I vote 5.13, especially since you are doing labor and won't have nearly the repair bill as most of us.

YNOWGN

I have a 2011 JKU sport automatic, running 37's and 5.13s... at 70 on the highway in 5th, I'm running about 2200-2300. I can't say what you should do, I just personally think I would go with 4.88s if I were still on 35's. And as mentioned above, I wouldn't regear a D30, I'd find a rubi takeoff or go with a UD44 or PR44.

karls10jk

I'm around the corner from you, running 35's on 3.73's but I'd choose 4.56 with my 6 speed and the general consensus is to go an extra step for autos so 4.88.

Josephtate

Thanks for all the input from both sides between 5.13 or 4.88 👍🏼 I forgot to mention that in the future there is a possibility of 37s. I’m leaning towards the 5.13s but I’m not 100% sold yet. My main concern isn’t strength of the D30 at this moment, more of what my daily driving will be like between both gear sets. Off road the 5.13s would clearly dominate for the way I wheel.

As far as an axle swap goes, I would love to swap one in from a Rubi especially if I can pick up the electric locker with it however is it direct bolt on as far as the rest of my suspension goes? I know the D44s are a good chunk larger than the D30s. I’m not very concerned with the strength/weakness of a D30 at this moment as I’m not one to put it in 4L and stomp it for testosterone/ego/etc. as previously mentioned lol I’m much more concerned with conquering my obstacles with skill vs sheer dumb luck and heavy throttle. I.E. use the winch if necessary and climb with thought in it. But this is all a conversation for another topic lol

resharp001

Honestly, the difference between 1 gear set it so minor. It's there, but you're only talking 100 rpm or whatnot at the high end. My understanding with the 3.8L is that it's pretty forgiving at higher RPM, and it's hard to undergear that 3.8L. If blowing up a D30 is the least of your concerns, I'd lean 5.13. Either 4.88 or 5.13 is going to be a big improvement over what you have it in it now......but it's probably not worth agonizing too much over which one to go with, especially since you are doing the work and the cost is primarily time as opposed to the financial commitment most have.

karls10jk

If you're going bigger then certainly gear higher. As far as the d44 goes- it all bolts right up. If I had the money, I'd be buying the ones I've seen locally. I think there was one that was already regeared even. The d44 has the same housing as the d30, the difference is the size of the ring and pinion.

Chainringtattoo

I live in Colorado and I would not want anything less than 5.13s on my 2007 Rubicon with 35s. If you are thinking about going 37s one day you really want to go 5.13. At 75 mph I see around 2700 rpms. The speed limit in Utah is 80 and Moab is only 90 minutes from here for me but I can't really do 80 in a 2 door with 3.5" lift. Too much vibration from the well worn Nitto Mud Grapplers and usually too much cross wind. 77 MPH is about all my rig is really good for. Even with 5.13s it's a dog on the climbs on the highway. Hope this helps some.