Regear questions with an AX15

In fear of being flamed: I have searched... and not found what I am looking for.

I have 31" tires on my 3.07 gears. I really do not like the drop in power I have experienced from factory, and I really would like to get back to a more factory-like power level. My math (linear scaling is around 3.4 which just doesn't exist) puts me at looking into 3.55 gears, but I had a conversation with a tech with Just Differentials, who recommended 3.73's or 4.10's which in 5th at 75 would put me outside the 2100-2300 RPM equilibrium I heard the 4.0 lokes to have on the highway. I don't think he took into consideration the .79 5th gear in the AX15, and figured I had AW4 like ratios. I have done some reading and searching, but nobody really seems to have documented what I am looking for. I am not changing tire sizes anytime soon, this is my DD/hunting jeep, and most people regear at 31" in anticipation of larger tires it seems. Is my mileage going to be that much worse at 3.73 vs 3.55? Has anyone on here done 3.55s and an AX15 with 31" tires?

31 inch tires at 75 MPH
RPMS Gears
2279.8 3.55
2395.4 3.73
2633 4.10

I ran 31s for a while on 3.56 stock automatic axles. It was cheaper for me to get a 30/8.25 from an automatic at the junkyard than regear my 30/35, and it ran well commuting at 70, think I got 15mpg. I have since gone to a 30/44 with 4.56s on 33s with the anticipation of going to 35s. If I were regearing and staying on 31s then go 4.11s, but it is really cheap and easy to swap in factory 3.56 axles.

When you were on 3.55 what was the power like? the JYs around here only have trashed XJs so I really am weary of buying axels and not having someone rework them...

I felt stock

I have found that the 4.0 likes to stay above 2300 or so on the highway so that even hills don't make you lose speed.I actually am going to 3.73 gears with 235/75 15 tires right around 29" or so.With my 4.6 Stroker she'll really haul ass!!!:eeks1:

xeroOTG said:

In fear of being flamed: I have searched... and not found what I am looking for.

I have 31" tires on my 3.07 gears. I really do not like the drop in power I have experienced from factory, and I really would like to get back to a more factory-like power level. My math (linear scaling is around 3.4 which just doesn't exist) puts me at looking into 3.55 gears, but I had a conversation with a tech with Just Differentials, who recommended 3.73's or 4.10's which in 5th at 75 would put me outside the 2100-2300 RPM equilibrium I heard the 4.0 lokes to have on the highway. I don't think he took into consideration the .79 5th gear in the AX15, and figured I had AW4 like ratios. I have done some reading and searching, but nobody really seems to have documented what I am looking for. I am not changing tire sizes anytime soon, this is my DD/hunting jeep, and most people regear at 31" in anticipation of larger tires it seems. Is my mileage going to be that much worse at 3.73 vs 3.55? Has anyone on here done 3.55s and an AX15 with 31" tires?

31 inch tires at 75 MPH
RPMS Gears
2279.8 3.55
2395.4 3.73
2633 4.10

Click to expand...

Your mileage will see no noticeable change between 3.55 and 3.73. Any difference there is will likely be very minor. If you are going to do the regear, I would do 3.73 or 4.10. If you do a lot of city driving, the higher ratios will probably net you slightly better mileage, as it is not having to work as hard when stopping and going.

If you are frequently driving 75MPH and up and there are not a lot of hills, 3.73 is probably a good place to be for stockish feel (a little extra gear to compensate for the added width and weight of the larger-than-stock tires). If there are lots of hills, maybe 4.10. MPG will likely not change much with any of these, so mostly it is a comfort thing. the 4.0 has no issues with 2600 RPM for a long haul.

After writing a novel, I will close with this. Most everyone that gears down is either happy with it, or wishes they went a little lower. I am yet to see someone who thinks they went too low. Not saying it doesn't happen, but it is very rare.

Good luck.

Had 33s and 307s it sucked
Went to 33s and 4.10s it was a good compormise
Went to 33s and 5.13s it was great off road, but too slow in the freeway
Finally went 35s and 5.13s, better on the freeway, but needed 4:1 tcase to get the crawl back to where the 33s were.

Short story long, use the grimmjeeper gear calc
Compare ratios side by side

Dont pay full price for a readily available ratio. Axles with 3.55s or 4.10s are cheaper than just a gearset

I would go with a set of 3.55 auto/4.0 axles... check the chapter classifieds in your area. They are incredibly common, probably 99% of XJs had that drivetrain and gear ratio.

Junkyards may not have em but there has to be at least one person parting out a 4.0/auto/4x4 XJ in your chapter.

Alright, sounds like a plan. Thanks for your feedback guys! I will look for some used auto axles.

I pulled a camper in the Mts with the AX-15, 3.55's, and 31's. It was okay. You won't stay in 5th gear much. Even my old 90 likes to stay around 2500 rpm at highway speeds. Axles out of an auto are the cheapest way to go.

I think you'll be happier with deeper gears. I've run a ton of different setups and driven even more, and if I were going to run an ax-15 and 31" tires I'd run 4:10s in the axles. Which can be found in the junkyard if you keep an eye out.

Re: Re: Regear questions with an AX15

Gold Cobra said:

I think you'll be happier with deeper gears. I've run a ton of different setups and driven even more, and if I were going to run an ax-15 and 31" tires I'd run 4:10s in the axles. Which can be found in the junkyard if you keep an eye out.

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So with 4.10's am I still at 15ish mpg on the highway? I realise that I do drive a chest freezer on wheels, but I at least get 15ish MPG on the highway. I am looking to lock one if not both axles... which brings up the question of what makes a good shop to do work on axles? There seems to be a bunch of shady looking places around here... this is my second Cherokee and I'm trying to do everything correctly. Lol.

I get 18mpg on the highway even driving it like I stole it with winter gas, with an auto and a tired motor... you are doing something wrong or something's worn out if you are getting 15.

I dunno about good shops, but if you are looking at a lunchbox locker it's something you can do yourself pretty easily. Full case lockers are another story, especially in a d30 front axle, you need to set carrier bearing shim packs to reproduce the original backlash/pattern and preload. It's not as complex as doing a full setup, you can definitely learn it yourself in a day or so with the right tools, but it's not something you can do with no measurements etc.

As for finding 4.10s in a junkyard, I know it's different out west but up here in the northeast, 4cyl XJs/MJs are incredibly rare in the yards. I have seen something like 3 4cyl MJs in the you-pull in five years of junkyard hunting, all were 2wd, and a single 4cyl XJ, which was also 2wd.

Rules of thumb for axles:
auto 4speed/6cyl: 3.55
auto 4speed/6cyl/towing package: may be 3.73 or 4.10, but mine (factory installed) is 3.55.
auto 3speed/6cyl: not even sure this exists, probably the 2.8 V6 in the 84-86s if it does, and don't know what ratio it would have. I forget what tranny they put behind those but it wasn't the AW4.
manual/6cyl: 3.07
auto 4speed/4cyl: 4.56 (very rare, 87-90 only as far as I know)
auto 3speed/4cyl: not sure what this would have, or if it even exists
manual 4speed/4cyl: 3.55 (a dog, 3.55s needed so it would do highway speeds without redlining!)
manual 5speed/4cyl: 4.10
I've heard rumors of 84-86 4cyls of various sorts having 4.56s as well, but never seen one to verify.

But basically 98+% of junkyard XJs/MJs are going to have 3.55s, most of the rest will be 3.07s, with a sprinkling of 4.10s. Reportedly a dealer in the AZ area really liked ordering 6cyl/autos with 4.10s but I obviously haven't been to the southwest to junkyard hunt so I don't know how true that is.

Re: Re: Regear questions with an AX15

kastein said:

But basically 98+% of junkyard XJs/MJs are going to have 3.55s, most of the rest will be 3.07s, with a sprinkling of 4.10s. Reportedly a dealer in the AZ area really liked ordering 6cyl/autos with 4.10s but I obviously haven't been to the southwest to junkyard hunt so I don't know how true that is.

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There are three Cherokees in the yards around here, all with 3.55 ratios, I called at lunch today. For some reason people let cars rot in their yards around here and think they are made of gold. One place wanted $500 an axel. I can get gears for both axles complete and installed for $500 more total and know that the bearings, seals and other stuff are good. As to getting 15 on the highway, pre lifted I was getting 22-26 on the highway on long trips. IDK, I always assumed that the increased rolling resistance and upgearing by running 31s was what was killing me.

It may well be - but did you change your speedo gear out or are you taking the fact that your odo is off by ~11% into account when you calculate your fuel economy?

And that is insane price wise, I pay $130 an axle unless the cashier is in a generous mood, then I pay $100 an axle.

Re: Re: Regear questions with an AX15

kastein said:

It may well be - but did you change your speedo gear out or are you taking the fact that your odo is off by ~11% into account when you calculate your fuel economy?

And that is insane price wise, I pay $130 an axle unless the cashier is in a generous mood, then I pay $100 an axle.

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Honestly no, I've been waiting to get a new speedo gear, I estimated my mileage a month ago by mapping my trip and guessing the distance. I havnt found anything for less than $400 an axel, and that was 4wp almost 100 miles away :/

kastein said:

It may well be - but did you change your speedo gear out or are you taking the fact that your odo is off by ~11% into account when you calculate your fuel economy?

And that is insane price wise, I pay $130 an axle unless the cashier is in a generous mood, then I pay $100 an axle.

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Here in Florida you get any 5 lug axle for $100 every day except 50% off days then it is,....50% off the original price. 6 lugs and up pay $135.

The LKQ I go to here in Jacksonville have plenty of XJ's with the 4.0L engine and AW4 transmission. About 30%+ are 2 wheel drive only. I have been keeping my eyes open for a D30 from a 2.5L to match the gears in the D44 out back for almost 2 years. During those 2 years, I have seen only 2 XJ's with the 2.5L and both were 2 wheel drive and sported the D35 axle.

I lucked out this summer. Purchased a 88 mj 2.5 aw4 4x4 with stock 4.56 for 450.

If you go the 3.55 gear install route, I have a set of very good used 3.55 gears for a D-30 high pinion, and d-35 that you can have for the price of shipping. They are going to the recycler on my next trip.

Just went through this. Get part-outs from an automatic. 3.55:1 is great for a daily driver on 31s with a stick, and they're really cheap and really really REALLY easy to find.

HP D30: FREE
29-spline C8.25: $80

I was planning on new hubs, U joints, ball joints, wheel bearings (rear), and seals anyway.

If you don't plan on going bigger it's definitely the way to go for a DD. If you want to go to 33s or do more offroading than highway driving I'd go deeper.