Hello, I've got a 2008 unlimited X that I'm about to throw 1 tons in, those axles come with 4.88 gears and I'd rather find a way to keep those in if I can. My current problem is my jeep is running 37" tires with 5.13's and I often find the need for a 2 1/2 gear on the steep roads that I live on. I was thinking about a super charger, but 4.88 gears would probably still need to be geared up if it's going to be running anything bigger than a 37, but what about an atlas or rubicrawler? Can you run those on the road and have the same effect as a really high regear for steep areas?

Edit: what I mean to say is, with 5.13's I only ever use gears 1,2, and sometimes 3.
With 4.88's, bigger tires, and a rubicrawler, would that even out the lower gear ratio?

 

Those won't make any difference on road, they are 1:1 just like the factory case. You should be at 5.38 with the 37's on a 2008

 

Thanks for your reply, I believe I read the rubicrawler can be engaged in 2wd

 

So can the atlas, but your gearing will be too low for highway speeds.

 

I was figuring with the rubicrawler engaged I'd have 1:2.71 + 4.88 gears on what I assume would be 38-40" tires, and when I'm on the highway I can shift the rubicrawler off and run in gear 3-4 without it, I haven't found anyone with a rubicrawler who does this but to me it makes sense. Wouldn't that be a 7.56 gear ratio? 4.88+2.71? Maybe I'm horribly wrong...

 

It would be multiplication 4.88x2.71 with 38" tires and 4.88 gears you would be turning 2085 rpm@60 mph with the 2008 and an automatic if you go to 2.71 reduction in the transfer case you would be running 5650 rpm @ 60 mph.

 

Transfer case isn't the answer here, although they sure are fun in the dirt! With a 3.8, the answer is power. The 42RLE is also a horrible transmission with slushy high gearing and guts that are about as big around as a loaf of bread pushing a fat 4x4 around.

You can get a couple of extra ponies from a programmer on a 3.8, and it can help. You can also move the shift points around a little to get you a little more on the hills. I didn't see too much about how you use the Jeep other than highway, but you will do fine with a 5.13 gear set, 3.8s love to rev. 5.38s aren't available for a Dana 30. 5.13 is a little on the weak side, but if you're not bouncing it around on the rocks in four wheel drive you'll be alright. If you are, stick with your 4.88s.

I've had a lot of experience with forced induction, and for the money you can wring quite a bit out of a 3.8 with a top notch supercharger and a good tune. They're less effective at high altitudes and can have issues with pinging, so that's worth considering. It's a fair chunk of change, but way less than an LS or Hemi swap.

As much as a supercharger is fun, sometimes your 3.8 has some miles on it and often they like to drink a little oil along with their fuel, which they also like to drink a lot of. If your 3.8 is worn out, then invest in a programmer and just plan on keeping your foot to the floor. If you like your Jeep and want to keep it for a long time, just start saving your pennies for an LS swap. Blow your 3.8 up, it's fun to drive that motor with no F's given, and the thing about that motor is that it'll slowly burn itself out, rarely ending in a catastrophic failure, they're funny that way.

 

He is putting tons in it so there are 5.38 gears available.

 

Correct, I'm putting in 1 tons that come with 4.88, I off road as often as I can on steep logging roads and often drive over creek beds and stumps that I fear will bend my d30 or break the pinion. I don't daily drive this jeep, and the max speed I go is 60, I often find myself in town going up long steep hills at 30-40mph when the speed limit is 45-55. I don't have the budget for a new engine like an LS or hemi, and I do already have a diablosport trinity. I'd like to be able to go at least up to 45mph in 4th gear with military surplus unimog tires and also have a set of 37's for off roading with. My hopes for the rubicrawler where that I could use it on the steep streets and hills that have a lower speed limit, and run in 4.88's without the torque from the rubi or atlas at 45-55mph. Does that make sense? If I got a super charger I'd also have to regear and I think eventually buy a new engine and transmission sooner than I hoped for without one. Jeepy's already got 150k miles on it.

 

simply remove the front drive shaft and drive it around in 4 low this will be the same thing as running an atlas or rubicrawler and you will see how slow you will actually be going and how high you will be revving that old 3.8.