Cherokeewaterfowler

When I lift my xj and throw on 31" tires will I be able to run stock gears or will I need to regear, if so what gears will work best? How much will it cost and is this something I can do on my own... is it simple?

Keep in mind its mainly my DD with LIGHT offroad use and light towing.

Thank you,


CW

VTJeep

If you want it to drive like it did stock, you'll have to regear. But with 31's there will be a very small difference and you will by no means need to regear. With 33's it becomes more apparent but many still go without. Stock the autos came with 3.55 and manual with 3.10. In order to stay stick the jump to a 31 would only require a 4.10 (one size up) and for the cost there's not enough gain.

XJlimitedx99

Regearing is expensive and something that is usually left for professionals. 4.10s on 31s with an auto would be perfect but it doesn't justify the cost. 4.56 with 33s is also popular.

If you can get your hands on the axles from a 97-99 four cylinder XJ then it will have a HP Dana 30 and a 29 spline 8.25 with 4.10s.

Fred/N0AZZ

Well no it is actually several gear sizes up and the difference is noticeable and would be very much so if towing. Some of the Jeeps with stock tires had a towing package and came with 3.73 gears in them.


If your sure that you want nothing more than a 31" tire "Ever" then you need a 4.10 gear for it. The 4.10 over the stock 3.55's will give better mpg and keep your trans from up/down shifting all the time on almost ground causing it to overheat. The 3.55's will give it a more sluggish feel on/off road.


Most all start with a small lift like a 3" and 31" tires I did and know a little about these issues, been down that road. I'm now at 4 1/2" and running 33's and 35's but when I bought my gears I bought for 33's a 4.56 gear bringing the 33" tires back to what the Jeep was with a stock tire and 3.55's. Gears are one thing most want to do only once, same for lockers (they should be installed at the same time no extra cost involved).


As for doing it your self and a first time project I would advise against it JMHO as what is involved. I paid a 4WD shop $200 for each differential to have the gears and lockers installed. I bought my gear set from Rustys for my D30/Chy 8.25 4.56 with the Master Install kits included for under $500 for the set. They were the same gears I have used in my race cars up to 1500 HP Drag Cars and 750 HP in Dirt Track cars.

Fred/N0AZZ

To hard to find in most areas and overall cost is to high to buy an unknown differential/gear setup when for $900 you can have all new installed.

jeep96cherokee

I'm running stock 3.07 gear (manual tranny) on 31's and I daily drive mine. I didnt lose enough power to make me want to regear. I can still take it on the expressway with no struggle at all. My MPG did suffer alittle bit, but my smiles per gallon is still high so its all good haha.

Regearing can be very complex and difficult especially shimming it and getting the backlash right. I wouldn't venture doing it yourself unless you or someone else knows exactly what their doing. When I graduate to 33s soon, I thinking about just swapping axles with 4.10s or whatever I choose.

jedijeb

Along the same lines as the OP, I have been thinking of 31s for mine. It has the 2.5l with 3:73 and 3spd auto, not overdrive. It is on 235s now which are right at 29" and has no problems, would it make much difference at 31" without re-gearing?

XJlimitedx99

yeah they're definitely few and far between. Some people get lucky though.

Metmknice

Older body XJ's with 2.5/manual had 4.10s as well. At least 87-up. Not too hard to find, usually.
For 31's I would definitely go 4.10 at least, if not 4.56. If you even think you might go taller than a 31, go 4.56 and call it good.
I very much disliked driving my renix XJ with 31's and 3.55 gears. Got a great deal on axles with 4.10's and never looked back. Probably would have gone for 4.56's had I had to pay a lot of cash to do an install.
My renix MJ 4.0L/manual and 4.56 gears on 31.5 tires averages 20+mpg in my typical DD commute.

VTJeep

I respect your decision because I know you know your ****, but I really dont think "need to regear" is totally accurate.. I did say that to be at stock configuration, 31's and 4.10's is needed, but for the cost of buying and installing, the jump from 3.55 to 4.10 is a huge cost for a tiny gain. To OP, why not get the 31's on and then decide if its worth it to regear. For the interim when towing keep it in 3rd and it will be ok until you decide.

IDK the output of a 2.5L but theres no way you wont notice the difference. Whether or not its enough to make you care is another matter. Im sure theres someone else running a 2.5 with 31's that would be able to give you a more honest answer, but theres plenty of people who run 33's on 3.55's (4.0L) without any real issue.. Not that its recommended.. Heres a visual aid to better understand what your doing when you dont regear.

93_xjcherokee

I'm on 32s with 3:55 gears and so bad I want to regear. I can do 65mph on the highway at about 2500rpm. I won't push my jeep over 3k rpm so I'm pretty limited

VTJeep

A big factor is rotational mass, spinning mass, call it what you will, heres the easiest answer I've found so far:

Shaving a pound from your tires is equivalent to shaving at most 2 pounds of non-rotating weight. That's PER TIRE, so a pound off each tire could worth close to 8 pounds of weight reduction. For wheels, the multiplier is closer to 1.6, so saving 5 pounds per wheel (20 total) would feel like a static weight reduction of 32 pounds.

Heres the link to article I just quoted.

Read Post 2

Betsy93

I'm running my xj with a 4.5 lift and 31 general tires with stock gears and I notice a little diffrence but it's fine on the highway and offroad but soon I'll go up to 33s and regear everything

VTJeep

Grabbers are heavy. 55lbs for a 31x10.50. If you go to a 33x10.5 or a 35x12.5 its 62lbs. and 65lbs. respectively.

XJlimitedx99

As much as I love this I dont think it really fits the discussion. Regearing puts more stress on spider gears, axle shafts, u-joints and anything else "downstream" of the diff because there is more torque being applied to the weight of the bigger tires. It also stresses the body more because the extra torque can twist everything up that much easier. It's only easier on the driveline components that are "upstream" of the differential, like driveshafts, tcase, trans, and motor.