jburg318

So, I know that this has been beaten to death, but I definitely want to regear as my XJ is a dog with the stock 3.07's I have in my D30 HP and Chrysler 8.25 with my 31" Cooper MTP's and 3" lift.

I really think I have her dialed in after a year of tinkering, troubleshooting, and replacing pretty much every sensor that has gone bad due to age.

The last 4 refills with mix usage (50% city/ 50% highway) have been on E0 summer blend (87 or 89 test) 17.8 (identified a slight vacuum leak on my one way check valve causing a rich idle), 19.3, 19.7, 18.9 mpg and I don't want to loose that economy as the main usage for the vehicle will be as a camping rig/ overlander/ and daily driver. I would really like to maintain my 350 to 400 mile/tank range. As anyone who has had this set-up before you know that getting off the line from a stop is not the most fun and although I drive like a grandma, I only use my 5th gear on the highway on flat terrain as overdrive.

My question is in regards usage and best outcome:
1) I don't plan on wheeling or getting away from 31's or a larger lift.
2) Based on the Grimm Jeeper gear ratio calculator, in order to get back to "stock feel" I would install 3.39 gears (My XJ is an SE that came from the factory with a NP231/AX-15 that came standard with 215/75R15 tires) - Obviously, I would go with 3.55 gears as this would just be the cheapest option moving forward.
3) If you alter the calculator and use a 225/75R15 tire with 3.73 gears and an AX-15 (a pseudo AX-15 tow-package set-up) the matching set of gears would be the 4.10's.
4) I plan on only adding bumpers, a cross brace/transfer skid, and a roof tent. Maybe a wench.

From everything that I have read, everyone suggests the 4.10's are perfect for a 31 to 33" tire with the AX-15 and a lot of people wish they have gone with that ratio; however, nobody really talks about usage and I am assuming people are mainly hitting the trails.

I am leaning more toward 3.73's as this would would seem to give a bit more get-up and go, but still keep the cruising RPMs a bit lower 2076 vs 2282 with the 4.10's.

I want some feedback as I don't want to spend the money on the regear and driveline upgrades and then automatically regret that I had not gone with the 4.10's.

Anyone have any thoughts or have similar usages and have done the regear and loved or hated it?

Thanks for your feedback.

Battle

I ran 3.55s with an ax15 with various 31x10.5x15s - from Treadwright Guard Dogs to Geolander ATs and Mickey Thompson MTZ. It was a really good combo - still had decent punch on the street (I guess a bunch of bolt-on parts helped too) and enough grunt in 4lo to wheel pretty well. I honestly got the most use out of that set up - wheeled a lot with it. This is also with a front skid plate, 12k lb winch, and steel rock sliders.

I now run 4.11s with 33x10.5x15s and it's pretty similar. What I like about having taller gears is being able to get 20 mpg while averaging 70-80 mph on road trips back and forth from the trail. If I need more power, I just downshift and let it eat for a bit. That said, I will probably go down just a tad to a 265/75R16, rebuild a set of axles with 4.56s and then run 33x12.5x15s with frame stiffeners.

I think in anticipation of the extra weight, 3.73s will do just fine for you, but agreed - your cheapest option is 3.55s by way of an automatic axle swap. That's still a noticeable upgrade - 3.07s suck. I still have them in my GF's two-door and even with 235/75R15s, I downshift a lot on road trips when there is road trip weight added. At least the 4.0 likes RPM (er at least mine do).

5-Speed

I did 3.73s on mine with 31's. It was the perfect combo. Plenty of power and still got above 20 mpg on the highway. Even when towing 3500 lbs, the Jeep did great, just had to keep it in 4th for hills.

00t444e

You can grab a pair of 3.55 geared axles out of an automatic XJ, swap them in and sell yours to recoup some of the cost, or keep them for spares.

TrailerTrash

My 98 has a 4.0/AX15 with 31s. It has 3.55s and it is a turd. Crusing speeds on the highway is ok in 5th but you gotta be booking to make it up any hill with a decent grade without downshifting. Most of my commute is done in the 45 to 55mph range with a mix of flat and hills. Alot of going between 4th and 5th is aggravating.

I wouldnt recommend anything taller than 4:10s. If you get lucky you can find a pair of axles with that ratio that will bolt right in.

XJlimitedx99

Auto axles with 3.55s would be a lot cheaper and probably easier than regearing. If I was in your position I’d try that first. If you decide you want more gear then consider going lower. You can always sell your axles or the 3.55s to make money back. Also, you’d have an idea of what the gear change feels like and would have a better idea if 3.73 would do it or if you should consider 4.10.

It would suck pretty bad to pay to regear to 3.73 just to wish you had done 4.10.

Okay XJ

What did you end up doing? I’m in the same boat and I’m thinking 4.10’s is the way to go

TrailerTrash

Do it. You wont be sorry

jburg318

I have not got around to it. I live about 15 miles away from East Coast Gear Supply and was going to meet with them before pulling the trigger. They have years of experience and are well respected across forums, so I am going to go with their suggestion. I spent some time looking for 4.10 or 3.73 axles with no luck locally, but I was leaning more toward the 3.73s as there are some people that have stated that 4.10s are too severe for just around town on 31's

5-Speed

You will be more then happy with 3.73s. I have a feeling if I went with 4.10s, I wouldnt have like it. But at the same time, if you even think you are going to go with 33s down the road, best to put in 4.10s now and live with it until the time comes.

Okay XJ

I am not as worried about trying to find used axles. I’m going to regear with lockers, axles, and trusses on the factory d30 and 8.25 because that should be more than enough to cover what I’m going to do and if not then I can always sell them and get some tons

SatiricalHen

I have a Comanche with an ax-15 and I swapped in a 3.55 8.25 because I broke the d35. It's a 2wd street vehicle that is on street tires and they're like 28-29" range at stock height. It is great around town and has plenty of power on the highway. I can get on the highway and never touch the shifter after putting it in 5th. It pulls grade very well. The past few tanks city and highway I've been getting 18 mpg. When it had 3.07s and 30" tires I got 21 mpg twice and usually around 18 mpg. To me the 3.55 made driving it more enjoyable and I didn't see a hit in mpg from what I can tell average driving. I have enjoyed being slightly over geared.

My auto Cherokee on the other hand. I planned to stick with 33" tires so I went with 4.56 gears and now I'm on 35s and it has to downshift more than I'd like. So in my experience I prefer over geared vs under geared.