Best Gears For 31" Tires???

[escout]

So, I've recently looked through a bunch of gear vs tire size charts (as well as done the math myself) to decide what would be a good choice for ring and pinion in my XJ. Based on the setup I want to run, 3.90 gears seem like the best option, but it's no secret that they don't exist as an option for the Cherokee (at least I've not been able to find them). If anyone is running 3.90's, I would like to know. I live at 7200' elevation and stock gears won't cut it, especially in the mountains.

This is my DD and I need it to get reasonably good mileage. I frequently run 70mph and drive quite a bit of in-town too. If 3.90's don't exist, what's my next best option? 3.73's?? or 4.10's?? Any recommendations for good gear brands for these axles?


Additional Info:
2000 Jeep XJ, 4.0l, AW4, np231j
3" RC Lift and 31" Tires
(d30 low-pinion, standard-cut, 27-spline, 3.55) FRONT AXLE
(Chrysler 8.25, 29 spline, 3.55) REAR AXLE

Thanks!!
atriot:

 

[WAZZUXJ]

4.10's if you never plan to go bigger than 31's. If you think you will someday bump up to 32-33's then go 4.56's.

 

[Outlander560]

4.10s

 

[djlarroc]

i did 4:10s and am running 245/75 (about 31").. wish i had run 4:56. Climbing hills, it would be welcome, and I don't normally drive my jeep over 75 anyway. I'm getting an avg of 16.5 mpg city/hwy. i have a 4.0 auto 200k.

 

[OutkastBoss]

4.10

I got 4.56 for my intended 33's I never bought yet.

I have been Daily driving 4.56 on 31's for over a year and its okay but too low imho .
I don't ever go over 5mph over , if I was a speeder It would bother me on the highway .
Of course on the low end its a blast to drive and jumps right out into traffic and gets up to speed which is helpful in STL .

Off road I haven't used 4Lo In a long time , even playing in the rocks at Winterfest so it pulls hard .

33's coming soon , or so i tell myself .

 

[Johnnie Walker]

4.10's min. , if you can find a 4cyl XJ, most likely will have a pair of axles that are 4.10s some rare ones have 4.56 with autos. I have personally NEVER seen a 4 cyl. auto.

 

[blondejoncherokee]

leave em stock and dont waste money on 4.10s if you are planning on 31s. once you go from 31s to 33s/35s then just go 4.56 or 4.88.

just saw you live in the mountains. I ran auto/4.88s/31s for like 6 months at one point. it was totally doable. I say 4.88 regardless in case you ever go to 33s or bigger.

 

[WB9YZU]

Yup, a set of 4.10s would be outstanding and not too bad on the gas.
Look under newer 4 cylinders with 8.25's.

 

[Devilfrog]

You really want the 4.0 to sit at 2000rpm range at cuise for best mileage. 3.73 form grands or newer TJ's or 4.10's feom a 4cyl would be my choice for a 31 inch tire. I'm going to toss a grands rear my MJ when I find one.
4.56 would have the motor screaming on the highway, and actually decrease mileage.

My XJ runs 33's and 4.56 and I'm still rolling in the 20-22mpg range on the highway. Town is lower at about 17mpg average. Highway cruise rpm is 2000-2500 from 55-65 mph (80-100km/h)

 

[blondejoncherokee]

Ya I missed the good mileage part. I just feel like in a year or two of wheeling, and youll be on 33s or 35s and youll wish you had gone deeper

 

[escout]

Cool, sounds like 4.10's might be the way to go. Does anyone have good experiences with a certain brand of R&P?

Ya I missed the good mileage part. I just feel like in a year or two of wheeling, and youll be on 33s or 35s and youll wish you had gone deeper

Lol yeah, I totally understand that. I've already got the disease when it comes to going big on tires and builds.....Trying to go a little more modest on this build since I already have my dedicated crawler on 37's

 

[blondejoncherokee]

good call. ya i want to eventually get a stocker and just do like 2" BB and 31s and leave my crawler for the harder stuff and have something for everything in between...well in that case, go 4.10

 

[Frank Z]

Take the guess-work out of the equation. Go to the following link and do a side by side comparison of the various gear ratios and see which get's you the closest to the stock powerband.

http://www.grimmjeeper.com/gears.html

 

[Red91Laredo]

I'd go 4:10 as well... I have 31's on my XJ that I drive everywhere and sometimes it'd be nice to have lower gears. The power at lower speeds is good (3.55 gears), but on steep hills at highway speeds more gear would be really helpful. If you're definately going to stay on 31's, 4:10 gears should be about perfect.

I've thought of finding a set of 4:10 axles to swap in, but want to run 32's in the future, so I'll probably wait and regear to 4:56.

 

[carves]

Just as a comparison /info

I've recently gone to 4.10s ... and only running 30" tyres.

Quite happy with the result ... Better driveability and negligible fuel use variation.

 

[escout]

I finally got the cash together to do the build I wanted on the Jeep and all went well. I ended up putting 4.10's in on 265/70/r16's (right about 30.6"). I can tell that it's slightly over-geared for the tires, but considering the extra rotating mass of the tires, those gears seem prime! I would have regretted 3.73's.

At 65mph I run 2250 rpm
At 72.5mph I run 2500 rpm
At 80mph I run 2750 rpm

I've only run a few tanks of gas through, but so far I'm averaging about 17 mpg. That's on a mix of city and 70 mph highway driving.

 

[Anak]

Thank you for posting your results. This thread is one that I looked at a few months ago. I appreciate seeing the conclusion.

 

[markw]

4.56's would be about 100rpm higher at highway speeds and give you more grunt out of the hole or when wheeling. Once above 31" tires 4.10's are pretty much a waste of time.

 

[Rod Knee]

You live in Laramie, elevation 7000 ft above sea level. 4.56 will not be overkill at all, especially if you ever tow. I have 456 on 31's and they are just fine unless you want to go much over 65 mph for extended periods of time.

 

[Tony_SS]

Take the guess-work out of the equation. Go to the following link and do a side by side comparison of the various gear ratios and see which get's you the closest to the stock powerband.

http://www.grimmjeeper.com/gears.html

This is a great chart.

It seems that using 4.10's and a 32 (31.7") tire puts the rpm back to OEM range, which if great for us guys who DD. I don't want to be spinning sky high at 70mph, so that is my plan. 4.10 with a 32 in tire.. that will work well for me on and off road. If I was in a higher elevation like the OP, I would have done the same as him.. 4.10 with 31" tires to give it a little extra grunt.