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Sooo I have a 3.73 rear end, changes my tire plans???

10K views 88 replies 29 participants last post by  SecondTJ 
#1 ·
Thanks to this forum, I have learned that my 2015 JKUR is a 3.73 instead of a 4.10 like I thought all Rubi's supposed to be. Bought the rig back in June and it has 255/75R17s KO2s on it. Was wanting to go up to a 285/75R17 in either the Patagonia or Nitto Ridge Grappler.

So how will the heavier tire effect performance? I know I'm gonna take a hit, but need the brain trust to help me figure out how much of one. Is the brides daily driver, we are in South West Texas so mostly flat, but we do have some hills, we are headed to Moab in March, but not planning on making the tire change till after that.

So what am I looking at???
 
#2 ·
It will do the 285's OK. See what I have. The 4th gear in the auto is 1;1 and will put your freeway RPM's where 4.56's would be. For off road the 4;1 transfer case will do it. You may want to regear in the future to 4.56's.
 
#3 ·
285's will be fine. I run them with no issue. I do run a Pedal Commander and used Jscan to recalibrate.
 
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#5 ·
you will be good with 3.73's and 285's
 
#6 ·
Thanks to this forum, I have learned that my 2015 JKUR is a 3.73 instead of a 4.10 like I thought all Rubi's supposed to be.

The forum should have also informed you that the Rubicon came stock with 3.73 if it had an automatic with 4.10s as an option. If the Rubicon had a manual transmission, then it came with 4.10s.
 
#10 ·
That is exactly what happened. I like a lot of people apparently was not aware that 2012 onward Rubicons with auto transmissions were coming with 3.73 instead of 4.10, unless the dealer or buyer specifically ordered them with 4.10.

Learning is fun!!!!
 
#8 ·
For a daily driver it should be fine. If you ever plan on going back to Moab on the new tires, then you should regear to 4.56.
 
#11 ·
I noticed from your garage that you had the Nitto Ridge Grapplers in 285/75R17 on your rig before up sizing, what are your thoughts on them? I'm leaning towards them pretty strongly at this point. I have a set on my work truck (F250 Diesel FX4) and so far I'm liking them, but wonder how they perform on a Jeep doing Jeep things
 
#12 ·
Before I regeared I was running the 285/75/17s nittos with 3.73s. It was fine for about a year then I got tired of all the hills where I live.

You said it is mainly flat so I think you’ll be fine. Besides I would much rather have 3.73s and 34” tires with their performance than 3.73s with stock tires.


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#13 ·
how about going with some 255/80-17's nearly as tall as the 285/75-17 (I have had both) and lighter by a fair amount. I would do the taller tires before going to moab instead of after. I can tell you 255/80-17 Falkens are really suited for Moab.
 
#18 ·
None of us can honestly answer your question. I had 295/70-17's which is smaller than 285/75 with my 3.73 and I hated it. Went to 4.56 and loved it... Now I've got 35's on the 4.56 and it's decent, but I wish I had 4.88.

Believe the hype... Gearing is everything.
 
#22 ·
This discussion has gotten me to thinking 4.56 in the somewhat near future.

Living in west Texas you will not feel the loss of altitude horsepower. As stated you have the 4 to 1 transfer case and that will help a lot off road. Everyone has a different opinion. Having the 3.6 is a big difference from the 3.9. This is in your favor also. I had a 3.9 with 3.73s and 33s and would turn off the overdrive to run down the Colorado Highways without the constant torque converter unlock and downshifts. Wish I had lower gears.

I am currently running 4.10 with 37x13.5r17s and the 3.6 and it does fine on and off road in my opnion.

Disclaimer...I have 5.13s in the shop waiting for time to install.
Thanks for the info, the 255s KO2s on it now were brand new when we bought the rig and I just couldn't justify the cost of new tires at that time, with the cost of new bumpers, winch, etc. and I thought I had 4.10 so no worries when I got ready to up size!!! LOL
 
#20 ·
Living in west Texas you will not feel the loss of altitude horsepower. As stated you have the 4 to 1 transfer case and that will help a lot off road. Everyone has a different opinion. Having the 3.6 is a big difference from the 3.9. This is in your favor also. I had a 3.9 with 3.73s and 33s and would turn off the overdrive to run down the Colorado Highways without the constant torque converter unlock and downshifts. Wish I had lower gears.

I am currently running 4.10 with 37x13.5r17s and the 3.6 and it does fine on and off road in my opnion.

Disclaimer...I have 5.13s in the shop waiting for time to install.
 
#21 ·
Think of it, the 8 speed has a 1st almost 5:1. So the JL Runicon has 4.10's with a 5:1 1st. You can't re gear a JK to match that.
 
#24 ·
I have 4.5 BS on my Icon's and no rubbing and look great.
 
#34 · (Edited)
Everyone has their own standards, their own version of what is acceptable. For every person on here saying that 3.73 gears are fine with 35" tires you can find at least one person saying the opposite.
My experience is that 3.73 gears were too tall even for the 32" tires ours came with stock (JKU, 6 spd manual). I could drive it, but it didn't drive well. 6th was barely useful. And cruise control on the highway was useless as it could maintain speed up hills in 6th gear.
I could not imagine running the same 3.73 gears and 35" tires. But again, different people, different ways they use their Jeep, and different standards as to what is acceptable performance. In the end, it is up to each individual. The only positive to it is that re-gearing is fairly inexpensive, typically around the cost of a set of 5 of those 35" tires. And the re-gear shouldn't wear out. So if you need it, do it. If you don't, don't. But it is really rare to hear someone who re-geared say they wish they hadn't. Most of the people saying it isn't needed are people that haven't done it.
 
#36 ·
So if you need it, do it. If you don't, don't. But it is really rare to hear someone who re-geared say they wish they hadn't. Most of the people saying it isn't needed are people that haven't done it.
I have zero doubt that gearing would improve the acceleration and overall performance. The need for it is a completely different story. I don’t need it for how/where I drive my Jeep. It’s like asking a 392 charger owner why they didn’t buy the Hellcat. Well .. the 392 is good enough for a lot of people. No knock on the Hellcat owners. It’s just not a need or a want for some. To say it’s necessary on the other hand is completely ridiculous.
 
#35 ·
With the 6 speed stick the 5th is 1:1 and you can run that with 3.73's and 33-35's on the freeway all day. My auto 4th gear is 1:1 and that's what I do many times. 1:1 is great Cuz there is no gear reduction to cause heat and it's strong. But, a re gear is really the way to go. IMO, a re gear actually puts more torque to the tires where running in a 1:1 trans gear allows more engine RPM but not the torque build that a lower diff gives.
 
#37 ·
...... IMO, a re gear actually puts more torque to the tires where running in a 1:1 trans gear allows more engine RPM but not the torque build that a lower diff gives.
It all comes down to gear choice.

3.73 gears at 70 mph running 35's, 5th gear

Rpm = 2507
Torque = 933 Ft of torque to the rear wheels

4.10 gears at 70 mph running 35's, 6th gear

Rpm = 2177
Torque = 809 Ft of torque to the rear wheels

4.56 gears at 70 mph running 35's, 6th gear

Rpm = 2421
Torque = 900 Ft of torque to the rear wheels

4.88 gears at 70 mph running 35's, 6th gear

Rpm = 2591
Torque = 964 Ft of torque to the rear wheels
 
#46 ·
Well gang,
I have the exact same model/year/engine/trans as the OP. Yep, as has been stated, if the 2nd gen of the Rubicon was starting out on the assembly line as an stick, then the Jeep was to be fitted with 4:10 gears. But, if it started life as an automatic, it was to be fitted with 3:73 gears. But, if you (the dealer or, a private person that ordered the Jeep) wanted both an automatic AND 4:10 gears, the line was happy to change from 3:73 to 4:10 gears, for a measly $600 back then. I have no idea what it costs now for a line change.

Ok, enough history on Jeep building. Yes, our '15 Jeep JKUR was outfitted with the automatic AND a set of 3:73s. I'd know the performance of that engine, its acceleration, hill and grade climbing capabilities and more with all the bone stock wheels and tires. I knew EXACTLY how it performed in all given driving scenarios.

In outfitting our new Jeep with a leveling kit and some Mopar HI-top fender flares, I knew it was time for some 315 tires which, as most of you know, are the same exact tire as a 35". So, on they went. A set of Hancook Dynapro 315 x 70 ATs on 17" x 8" American Racing Anson Off Road Alloy wheels.

Well, knowing the performance of that jeep as well as I did, I knew right off the bat, that EVERY driving scenario was hindered. Now, I lived with this set of operational conditions for about a year. Finally, I decided to re-gear. I knew that going to 4:10s would be marginally better so, I opted for 4:56s.

From the very first take-off, I realized I'd not only got BACK all of what I lost in acceleration but, gained a tad more than stock. Now, a "tad" is subjective. The thing you have to realize is, in a situation like mine, EVERY single time you accelerate from a stop sign, stop light etc. you use a certain amount of throttle. If you know and retain just how much throttle is used from a stop sign, going up hill, with stock gearing and tires, then throw a set of larger tires on making any compensation, YES, you KNOW what you lost in acceleration WITHOUT using MORE throttle.

And, just like take-offs, climbing grades and using specific gears, was also labored with larger tires and stock gearing. But, with the change to the 4:56s, I'd gained back my hill/grade holding capabilities with NO ENGINE MODS what so ever. Every single take-off, every hill, every grade etc. is now easier or, I should say, less throttle is used to cover the same ground, at the same speed.

This is one of those things that you have to know your vehicle and how well it performs, BEFORE any changes to really gain insight as how it performs AFTER the tire size change.
Scott
 
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#48 ·
Yep, as has been stated, if the 2nd gen of the Rubicon was starting out on the assembly line as an stick, then the Jeep was to be fitted with 4:10 gears. But, if it started life as an automatic, it was to be fitted with 3:73 gears. But, if you (the dealer or, a private person that ordered the Jeep) wanted both an automatic AND 4:10 gears, the line was happy to change from 3:73 to 4:10 gears, for a measly $600 back then. I have no idea what it costs now for a line change.
It’s more complicated than that.

All 2nd gen Rubicon’s started out (2007-2011) with 4.10, regardless of manual or automatic.

It wasn’t until 2012 that the Rubicon’s automatic started coming with 3.73 standard and 4.10 optional. Back then it was only a $50 option. Jumped to $195 in 2013 and $495 in 2014. Continued to have an annual increase until it peaked at $695 in 2018.

Today there is no upcharge on the 3rd Gen JL Rubicon, Jeep went back to having 4.10 standard regardless of manual or automatic.
 
#47 ·
I feel fortunate and must have been extremely lucky as I have had 3 jk rubicons all with 4.10's and 35's and autos and never once felt I was under geared. I have enough giddyup and go to get out of my own way and can squeal the tires at will. I have towed a bigger camper in Montana where it is either going up or going down and towed a camper thru Utah and Colorado without any issues. I personally think I would be fine with 37's and 4.10's. I have driving lots of fast cars and motorcycles but I still think my jeep gets up and goes plenty fast.
 
#49 ·
IMO this is the difference between driving an auto and a manual. With an auto people are just not as in touch with the performance aspects of a vehicle. And to compound this lockup converter technology masks it even more. There is normally a 150-200 rpm drop when the tranny locks up. This is basically given you 2 different rear end ratios. When locked up it is 4.10's, when not locked up it is like have 4.56's.

That is also the reason why you see the recommendations of 4.56 for auto's and 4.88s for manuals.

FYI: I am not putting down auto's. For me auto or manual all comes down to gear spread. The 6 speed manual is much better performance wise than the 5 speed auto. If the 8 speed auto used in the JL was available in 2014 I would of opted for it. I also think coming up soon this will be the mod to have with our older 3.6's.
 
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