Differential ratio best for 31x10s

This has probably already been answered ad nauseam, so I apologize if it is already here in some convenient place.

My rig is a 2001 Classic (not quite a Limited, but upgrade from a Sport).
4.0L Power Tech plus K&N cool air intake and a Borla stainless smooth bend cat back.
Aisin Warner 4 speed automatic mated to Command Trac Part Time 4WD NP231 transfer case.
Dana 30/Corp 8.25" differentials w/3.55 gears.

I installed Good Year Wrangler Duratrac 31x10.5x15s on Keystone Daytonas along with a Rubicon 3.5" superflex lift, track bar and steering stabilizer. Additional 1" coil spacer to offset the heavy bumper. The corp 8.25" is tilted up 2.5 deg. to align with the 1" Skyjacker dropped xfercase.

The front and back WARN bumpers along with a WARN M8000 winch add some additional poundage.

Now at highway speeds, say 60-70 miles per hour I notice it is very easy for the tranny to slip back into 3rd gear even on mild hills to the point of being annoying.

I want to have the diffs and xfer case rebuild, just because my ear picks up some gear howl anyway, should I look at changing up the ratios at the same time?

What experience do others have?

Thanks in advance.

D.

If you are already gonna have the axles apart I would suggest regearing while they are apart. Most agree that 4.10s are perfect for 31's and if you plan on going 33's in the future just go ahead and get 4.56's. Or you can do what I did and pick up a 4.10 ford explorer 8.8 rear axle and a D30(also 4.10) from a 4cyl XJ and swap them in.

FlexdXJ said:

..Most agree that 4.10s are perfect for 31's and if you plan on going 33's in the future just go ahead and get 4.56's. Or you can do what I did and pick up a 4.10 ford explorer 8.8 rear axle and a D30..

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I think it's unlikely that I would move larger than the 31s, I want to maintain the vehicle as dual purpose, highway/terrain. Performing a swap seems to much of a nuisance for me, would need to unbolt almost everything that was just done with the lift kit

If I re-gear this set to 4.10s and replace the bearings and seals, then I'll have a set of differentials that should last me another 130,000 miles.

Thanks for the post.

I put a lot of miles on my vehicles and liked having stock 3.55's with 30's and 31's. I got 19-20pmg. I am now driving on 3.55's and 35's and am looking forward to getting 4.56's installed, but seriously, its doable as is, but mileage is around 16, and no overdrive...

My XJ is also a DD and I'm on 4.5 with 31's because of lack of funds to regear axles, Just do some engine mods such as a, oversized TB, header, Ignition kit. it'll help when pulling hills and such.

I went 4.10s w 33s If I could do it again I"d go 4.56s the 4.10s compensate for the tire size diff ie the speedo is a hair fast but w added weight and drag the 4.56s wouldve been better choice

4.10s and 31s will be very nice im runnin 4.10s and 33s its not bad at all but if i wanted to spend the cash id go 4.56s. i prefer to be one ratio over being stock performance again so 4.10s/31s is good and 4.56s and 33s is good

very clean xj buddy

Costs a hell of a lot more to get regeared than it does to buy a pregeared axle. If you can find an hp d30 from a 4cyl XJ at the scrapyard it will run you 100-200 dollars, it costs $260 with member discount for the parts to install 4.10s in an hp d30... and that's not including tools (if you do it yourself) or gear setup labor (if you have it done.) Plus this way it's bolt-in and when you are done you have a spare axle that you can sell or regear to an even deeper gear while daily driving the jeep still.

Same goes for the 8.8, you can pick one up for 100-200 dollars and prep it to go in, keep or sell your old axle.

4.10s are easily achievable with junkyard/stock parts, any higher ratio will almost certainly require regearing which requires skills/tools or deep pockets.

iceman2001 said:

I think it's unlikely that I would move larger than the 31s, I want to maintain the vehicle as dual purpose, highway/terrain. Performing a swap seems to much of a nuisance for me, would need to unbolt almost everything that was just done with the lift kit

If I re-gear this set to 4.10s and replace the bearings and seals, then I'll have a set of differentials that should last me another 130,000 miles.

Thanks for the post.

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Your welcome man. See below for the full explanation of why I said to swap axles.

kastein said:

Costs a hell of a lot more to get regeared than it does to buy a pregeared axle. If you can find an hp d30 from a 4cyl XJ at the scrapyard it will run you 100-200 dollars, it costs $260 with member discount for the parts to install 4.10s in an hp d30... and that's not including tools (if you do it yourself) or gear setup labor (if you have it done.) Plus this way it's bolt-in and when you are done you have a spare axle that you can sell or regear to an even deeper gear while daily driving the jeep still.

Same goes for the 8.8, you can pick one up for 100-200 dollars and prep it to go in, keep or sell your old axle.

4.10s are easily achievable with junkyard/stock parts, any higher ratio will almost certainly require regearing which requires skills/tools or deep pockets.

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Great post replies, thanks.

98stocker said:

I put a lot of miles on my vehicles and liked having stock 3.55's with 30's and 31's. I got 19-20pmg. I am now driving on 3.55's and 35's and am looking forward to getting 4.56's installed, but seriously, its doable as is, but mileage is around 16, and no overdrive...

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Did you have much grief with the tranny stepping down too often with the 31s? You must with the 35s --yes?

clean4drxj said:

My XJ is also a DD and I'm on 4.5 with 31's because of lack of funds to regear axles, Just do some engine mods such as a, oversized TB, header, Ignition kit. it'll help when pulling hills and such.

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Hey clean4drxj, did you mean to say you're on 3.5 and 31s? I have considered sticking with the 3.54s or maybe just move up to 3.73s, by the math the 3.73 would be the right ratio to make up for the difference between the stock wheel circumference and the 31s, it's what is recommended by the local Jeep mod shop (www.JustJeeps.com), but have concerns on making up for the added weight of wheels, bumpers, etc, and the added rolling resistance of the aggressive and wide tread. Perhaps a 4.7L Stroker with lots of low end torque then the 3.54s look sweet

harleydrifter said:

I went 4.10s w 33s If I could do it again I"d go 4.56s the 4.10s compensate for the tire size diff ie the speedo is a hair fast but w added weight and drag the 4.56s wouldve been better choice

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Sounds like you'd agree that 3.74 would be right for the rotation, but 4.11 for the extra weight and rolling resistance, or maybe even 3.91 to split the difference ... hmmm, might be on to something there.

1990heepxj said:

4.10s and 31s will be very nice im runnin 4.10s and 33s its not bad at all but if i wanted to spend the cash id go 4.56s. i prefer to be one ratio over being stock performance again so 4.10s/31s is good and 4.56s and 33s is good

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I hear you, for the reasons above.

filterbiz said:

very clean xj buddy

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Thanks guy, the interior is extremely clean as well, have a dark grey cloth interior installed. I'd still like to get my hands on an overhead console from a Limited and still need to fabricate some rear bench headrests, always going for the clean factory look.

kastein said:

Costs a hell of a lot more to get regeared than it does to buy a pregeared axle. If you can find an hp d30 from a 4cyl XJ at the scrapyard it will run you 100-200 dollars, it costs $260 with member discount for the parts to install 4.10s in an hp d30... and that's not including tools (if you do it yourself) or gear setup labor (if you have it done.) Plus this way it's bolt-in and when you are done you have a spare axle that you can sell or regear to an even deeper gear while daily driving the jeep still.

Same goes for the 8.8, you can pick one up for 100-200 dollars and prep it to go in, keep or sell your old axle.

4.10s are easily achievable with junkyard/stock parts, any higher ratio will almost certainly require regearing which requires skills/tools or deep pockets.

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I appreciate the suggestions on cost savings, and don't wish to make it sound like I have more bucks than brains, but do have more bucks than time. I can't stand touching something without fully rebuilding it with all bearing, seals, gears if necessary. Even with a wrecker's axle, I would still rebuild it --just really hate doing a job twice, although I do have two kids :roflmao:

FlexdXJ said:

Your welcome man. See below [now above] for the full explanation of why I said to swap axles.

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Completely understood.

That's a lot of work to simply avoid the 4th to 3rd downshift annoyance... just slip it into 3rd gear and switch to tow mode and leave it.

I ran 31s with a 3" lift for 10 years and I'm now running 31s with a 5" lift.... and been doing both for over 15 years...

IMO the best overall street / highway ratio is 3.73... I have 3.73s in my 2002 Chevrolet Express 1-ton van (8.1L) and my new 2010 Grand Cherokee SRT (6.1L). However I would go with 4.10s if I was regearing my XJ to make "D" behave more like the "stock" 3.55 gearing while running 31s.

It doesn't hurt the trans to drive around in 3rd... been doing it for 20 years on mine.

.

4.56, do it once and be done with it. You have a lot of extra weight on your XJ and your tranny will appreciate the lower gearing. You will love the way it drives on 4.56

You might want to try National Drivetrain's web site they have a great chart that shows you RPMs at various gearing and tire sizes.

Gonepostal said:

You might want to try National Drivetrain's web site they have a great chart that shows you RPMs at various gearing and tire sizes.

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Great suggestion...

I think he'll find a nice chart illustrating the fact 4.10s will get him close to the "stock" RPMs... just like it rolled out of the factory.

.

bchulett said:

Great suggestion...

I think he'll find a nice chart illustrating the fact 4.10s will get him close to the "stock" RPMs... just like it rolled out of the factory.

.

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Hey, he's newish to this, he will learn! You are right on though. This is why i went with 4.10 axles instead of having mine regeared. Well, that, and the fact that the axles were locked when I bought them! If they weren't laying on the garage floor I would be even happier with them. I keep getting people who tell me that if I go lower(numerically higher) I will lose MPG's. I have oversize tires people. They don't seem to understand that part.

I re-geared to 4.11 several years ago when the XJ was still on 30"s. It was great fun to drive but I lost 1 MPG, at freeway speeds. When going to 31”s, the mileage when back to 19 MPG. As height and weight were added, the mileage dropped a little more.
When I installed 33"s, the mileage dropped another MPG. It still drives fine but 4.56 would be even nicer. BTW, I never intended to go bigger than 31"s either.
There are good reasons not to go bigger than 31"s; inadequate brakes being a big problem. I'm sure you have noticed that your XJ does not brake as well as it did when stock. Heavier tires, higher COG and added weight all take their toil. Stock, my XJ weighted about 3400 lbs, now, when loaded, it is around 4000 lbs. You have bumpers, a winch and who knows what, so it has gained quite a bit of poundage too.
My XJ was scary to drive, at freeway speeds, on 33"s, before upgrading the stock front and rear brakes. Even with well maintained brakes, with premium pads and rotors, it would not stop straight or very quickly.
ZJ rear disc and WJ or Vanco fronts should be on everyone's list when going bigger than 31"s, especially if street driven IMO.

Having the axles done by a professional should give you better peace of mind compared to wondering about the condition of a junk yard axle. The down time should only be a day or two, with planning.
Another advantage is being able to add lockers or limited slip differentials.
I have run a rear TruTrac and a front Ected for years and they do the job in everything except deep rock.

Agreed on never intending to go bigger than 31s... when I showed up here I intended to stay stock and just replace my leafs... got more lift than I intended... said "no bigger than 31s" ended up with some bargain 32s... then had to replace them and found 33s for cheaper than I could get 31s... :doh:

Mine actually stops pretty well, even with stock brakes and crappy rotors up front.

lazyxj said:

I re-geared to 4.11 several years ago when the XJ was still on 30"s... There are good reasons not to go bigger than 31"s; inadequate brakes being a big problem. I'm sure you have noticed that your XJ does not brake as well as it did when stock. ... You have bumpers, a winch and who knows what, so it has gained quite a bit of poundage too. My XJ was scary to drive, at freeway speeds... ZJ rear disc and WJ or Vanco fronts should be on everyone's list when going bigger than 31"s, especially if street driven IMO. Having the axles done by a professional should give you better peace of mind compared to wondering about the condition of a junk yard axle... Another advantage is being able to add lockers or limited slip differentials...

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Yes, will go the rebuild route on my existing D30 and Corp 8.25 Trac-Loc and go with the 4.11s.

Loud and clear on the braking issues. I find the need to keep a good distance back from cars, trunks, bicycles, and nerds on Segways.. (too soon?).

Question on the Vanco fronts, are these kits a collection of parts counter known parts or does one need to obtain rotors, pads, caliper kits from Vanco when service dictates?

Any opinion on the TeraFlex rear disk kit?

truely not sure why everyone says our stock breaks are inadequate, i could lock up 33s on my other xj no problem. my xj now cant lock em up now due to abs. i think everyone is too used to brakes on our new cars that u barely push the damn things and almost get whiplash if your not expecting the sensativity. its understandable that xj's are older tech and dont have great brakes but they work just fine. if my yota was able to lock up 37s on factory brakes then i gaurantee my jeeps brakes are just as safe with 33s