Don't worry, I highlighted the pertinent information that fusion 4x4 was trying to relay to you boys.

 

There is because people are over compensating.

Let me guide you through the math. In this example, we will use a 2012+ auto. At 70 mph, depending on gears (range is 3.21-4.10) a stock JK rpm is 1959-2502. With 37s and 5:13, it is 2800. Given a final drive of .83, the driveshaft is spinning at 3400. Most people I know, cruise faster than 70 on the highway. The driveshafts are not balanced for that rpm. In addition, they are not operating at stock angles, which further adds stress to the driveline. At 80, it would be spinning at close to 4k rpm. I'd pass the back seat for that ride.

We're not saying you can't, just be aware of the potential issues such as burning up pinion bearings or seals and cracked t cases. And with aftermarket shafts, it's much more difficult to manage vibrations due to weight and the introduction of u-joints.

 

Thank you. We don't make stuff up, it is publicly available technical data.

However, we all know someone who runs 40s on a D30 without issues, so it must be fine.

 

ok guys, I am not worried about the driveshafts because they are no longer stock and I have upgraded my u joints as well. I am fairly confident that my ds or u joints wont be a catastrophic failure. I just merely want to run as close to stock power as possible without it lugging as much on the street as it is doing. I am at roughly 2700 rpm at 75mph and my buddies 14 with 35's and 5.13 6spd is at 2900. I know I will gain back my acceleration power and maybe even gain mpg but I just want to be sure that 5.13 isn't going to kill my mpg.

 

Actually the stock driveshafts are much better at handling higher rpms. No matter how much you upsize the u joint it's still rated for 3k rpms. Just figured I'd let you know that regardless of what you care to hear.

 

Actually the stock driveshafts are much better at handling higher rpms. No matter how much you upsize the u joint it's still rated for 3k rpms. Just figured I'd let you know that regardless of what you care to hear.

 

I suppose it matters to some extent where you live and the terrain, but in my opinion anything more than a 4.88 with 37" tires will kill the MPG. For me in western PA with the hills, I actually get much better fuel economy around town bopping about at 40 MPH than I do on a highway at 65. I'm using 4.88 gears and 37" tires, and like I mentioned, I get better (17-ish) mpg around town, but if I set the cruise on 65 and run 5 hours to Rausch Creek I get 13.5-ish. Personally, I wouldn't wish for more gear in the axles in high range. What I'll likely do someday in the future, and with no feeling of schedule pressure, is swap out to a Rubicon 4:1 T-Case or an Atlas case for a slower/lower low range. I think that for me, I would have rather went to 4.56 gears and then an Atlas/Rubi case to make the low range what I would like. 4.88 is plenty low enough, if not a bit over geared on the roads for a 3.6L Pentastar with 37" tires from what I see.

I also don't go off road every weekend, so for every mile I will do off road with my Jeep, I'll do a couple hundred miles on roads. If you are like Chuck The Ripper and live right near a great off road area, you will likely desire the lower gear since your actually off road every weekend or two. So I would say it depends a lot on your local roads, flat or steep and hilly. And your frequency that you will be playing off road. You can sacrifice a bit of gear for better road performance, or sacrifice a bit of road performance for more off road based gearing. Or go Rubicon/Atlas case and get the best of both.

 

JKO tends to have a lot more knowledgeable people when it comes to technical info. Here is a timely post of someone whose transfer case blew up while driving. Follow the thread and you'll see it is fairly common and related to gear ratio and driveshaft. Just because you don't feel it, doesn't mean the damage isn't happening. A new t case is serious $$$. Replacing worn pinion seals and bearings is also expensive. If you search a little, you'll find leaking pinions are common following new driveshaft installs.

http://www.jkowners.com/forum/modifi.../328585?page=1

 

Heck I thought 4.88's with 37's was overkill, but I have a 2015 auto. I'm leaning towards 4.56's and 35.5's but if I go to 37's someday I won't bother regearing.

 

While this topic is floating around I'm hoping for advise as I've been battling a mild vibration. My story so far, I have a 3.8 auto and tyres which measure to about 34", 3-4" long arm lift and about 4-5* caster . After my regear to 5.13 I had no issues at all but did zero highway driving. After my first time on the highway about 800km round trip my front drive shaft shat itself. (cruising speed was only about 2550 rpm in overdrive). Ordered a new double Cardin 1310 drive shaft and removed the stock one while I waited. From here I started noticing a mild vibration when I would reach 2000rm in top gear, at this time I had no front shaft in so can only assume it's from the rear? Since putting new shaft in I've noticed my front pinion seal has started leaking which sucks. But I'm confused why I suddenly have this vibration that comes in at 2000rpm?

Is there any way to tell if it is caused from my rear drive shaft? I was thinking about removing it and going for a drive while in 4hi, does anyone see an issue with this?