Well, you are going to get a mixed bag, but that is something we actually did. 315 Duratracs, went to 4.88, then 37s. However, the Duratracs were only on for 2 days.

4.88 had revs at the high end even with 37s. Those Duratracs are at best, a rolling diameter of 33.5", but probably a little less.

Revs will be very high, probably around 2900 at 70. Personally, I would not want to run the engine at 3k+ RPM.

Now, for the good reason you shouldn't do it. Final drive is .83. That means that at 70, your driveshaft is spinning at 3600 rpm. Driveshafts are usually rated for 3k rpm at constant speed. That is for a few reasons. 1. Most machines balance to 3k rpm and not beyond. 2. Vibrations from u-joints. 3. It can begin to oscillate.

What does all that mean? Rapid wear of u joints, pinion bearings, and pinion seals. Likely a significant increase in driveline vibration (doesn't have to be the kind you feel).

If that doesn't make you think twice, take a look at pictures of cracked t-cases. Most happen on the highway. The above will explain why. A t-case failure is catastrophic and you will be lucky if the only thing damaged is the case - they can cause a significant amount of damage, including into the cab of the vehicle.

Sure, lots of people do it, just like putting 41.5" tires on stock JK axles. Just set your expectations so you aren't surprised if/when it happens.

Last thing I will mention is that even when the 37" MTRs went on, 4.88 still felt a bit much, but it rarely saw highway and when it did, the cruise was set at 70.

 

I appreciate the info. So...based upon your last comment, are you saying that 4:88 for 37s are too much for a rig that is a daily driver that sees in town and highway? Should I be looking at 4:56 for my purposes and being an auto?

 

Ok another 1st hand experience... My 2012 JK Sport Auto 305/70r16 Irok ND (32.8") went from 3.21 to 4.88s. I already had the 17" breadlocks in the garage and the 37x14r17 Irok were about to be ordered.

130miles of highway... And the ECU hated it and throw codes of I tried to do over 70mph for more than a few seconds. The safe max speed was 65mph... 60-65mph you're good.

Then I drove around town for another 150 miles. Around town and staying under 60mph you're fine.

I know my 37 Iroks are only 36.8" but I also run a AntiRock upfront so 65mph is about the max I do on the highway now.

 

I wouldn't be afraid of it for a short time, especially with stock drivelines and not much lift to screw up driveline angles. I only occasionally get on the Interstate with my Jeep so my 5.13's and 315's (mine were 34" tall when new) haven't caused an issue for the 6 years it's been like that. I mostly ran 2.5" RK springs and my drivelines are stock. Hell, I ran 5.13's and 32's for 6 months. Pretty sure I avoided freeways for that period. :wink2:

3.8L manual, BTW.

 

I only occasionally get on the Interstate with my Jeep so my 5.13's and 315's (mine were 34" tall when new) haven't caused an issue for the 6 years it's been like that. I mostly ran 2.5" RK springs and my drivelines are stock. Hell, I ran 5.13's and 32's for 6 months. Pretty sure I avoided freeways for that period.

3.8L manual, BTW.

The key to that is you have a MANUAL. The W580 automatic in the '12+ are computer controlled with very tight tolerances in the tuning and all its sensors. Its more sensitive to things than the '07-'11 automatic.

 

I ran 4:88's for about a year or so before I got my 37's. To be honest I loved it.

 

What was your set up? 2dr or 4dr; tire size; lift size; heavy or light rig; etc., etc.?

 

At the time I was in a completely stock JKU Rubicon. I knew the direction I was going to go with my build so I removed the rear bumper, replaced the front bumper, went with new axles new gears and trusses. Then flat fenders (Nemesis). THEN I went with the metalcloak gamechanger with ARB shocks, then 37's.

My Rig is light, no spare, aluminum fenders no rear bumper, light front bumper. Will be adding the Synergy complete cage here soon. But to the topic at hand gears and axles were prior to suspension and tires. 4:88's were fine, obviously you are running higher RPM's on the freeway but its not a big deal, gas mileage hardly took a hit maybe a mile or 2 a gallon but when you get the 37's its going down anyway so at the end of the day who cares.

 

If you have something like the AEV ProCal you can program the tire size and gear ration so the computer doesn't throw codes and such. My friend runs 35's Nitto Mud Graps..on a 2012 JKU Rubicon with 4.88. Zero issue for tens of thousands of miles.

Your RPMs at 70 will be high...but your fine for awhile.

 

That was with a programmer... 305/70r16 (32.8) and 4.88 is too big of a difference. The 315/70r17 (34ish) is better, but you have to find tune the size since advertised and actual differ. 35s shouldn't have an issue.

 

If you do decide to go the 4:88 route, I've got a brand new set in the for sale section.