I have a 2000XJ 4x4, 4.0L, AW4 auto trans, front D30 and rear 8.25; both currently with factory 3.55 gear ratio, 31" tires, and a NP231 T-case.

Going up hill sometimes is a drag, literally. It's a bit of a dog going up hill or with headwind. Even just going up an overpass on the freeway I can see the MPH dropping. With headwind it can be ridiculously sluggish to where I need to put it into direct drive sometimes; depending how far the wind is blowing.

With 31" tires Im under the assumption that 3.73 gears might get me back to a stock setup; almost equivalent to original 215/75R15 tires with 3.55 gears. I also read that unless Im off-roading frequently then 4.10 gears might not be worth it. People also say going 3.55 to 3.73 isn't worth it and should just go to 4.10. (But they never explain why) Im also never going to go up in tire size. Ive had 31s for over 10 years on stock 3.55s. I think it's time for a noticeable upgrade. The most off-roading I do is the beach; I do run into loose sand but Im 90% of the time on dry pavement.

I would appreciate some advice from people who have ran, or do run, 3.73s or 4.10s on 31s with 4.0L and auto trans. What are your experiences with this setup?

 

I'm running 4.10s with 31s, 4.0L and auto trans and it's darn nice. With the added weight of the larger tires, steel rims, bumpers, frame stiffeners, 2x6 rockers, etc, etc, I'm hard pressed to believe that any modified XJ could possibly weigh less than stock. That's the reason to run 4.10s instead of 3.73s. The difference between 3.73s and 4.10s is only a few hundred rpm at 60 mph. It cruises very well at highway speeds and has no problem keeping up with even the fastest traffic without feeling like I'm pushing it. 4.10s and 31s is very good combo. When I finally get around to installing lockers, I'll most definitely will not be going down to 3.73s, but have been mulling around going up to 4.33s. I live right where the rolling hills meet the PA mountains and having that extra little bit would be nice when pulling a trailer up to my cabin.

 

Play around with this calculator to see actual numbers: http://www.grimmjeeper.com/gears.html

3.73's will actually put you at a higher final ratio with 31's compared to stock size (I used 225/75r15) with 3.55's. I'm seeing stock is 2214 rpm at 70 mph, and 3.73's with 31's is 2122 rpm. Currently, with 31's and 3.55's, the calculator shows you're at 2020 rpm at 70 mph.

People say its not worth it to gear from 3.55 to 3.73 because you're only gaining 100 rpm at 70 mph, which still puts you 100 rpm below factory ratio. Going with 4.10's and a 31 inch tire will have the motor turning 2333 rpm at 70 mph, 100 rpm over factory ratio.

Considering any mods that are unaccounted for with strictly a tire size change, such as increased drag from a lift, extra weight from any gear being hauled, etc., the extra 100 rpm will be welcomed.

 

3.55 to 3.73 would be a pointless swap, I would go with 4.56 gears.

 

I don't have the extra weight of those thick plate stiffeners and sliders but I do agree 4.10 would be better to haul that weight around. Is this Jeep you mention your daily driver? Mine is and that's also one thing I want to consider when finding a gear ratio. Most of my driving is the city. When I do get on the freeway or highway out of town(rarely) I keep it at or under 70mph. At 60mph Im reading about ~1900-1950 RPM and at 70mph about ~2150-2200; in overdrive.

Could you check next time you drive and record what your RPM is for both 60 and 70 mph with your 31s and 4.10? Id greatly appreciate it.

 

That doesn't seem too bad. In the end 100 or 200 rpm adds up. 2000rpm for one hour is 120,000 total rpm; at 2300rpm thats 138,000 total rpm, for a difference of 18,000 added revs per minute. Thanks for the calculator link, Im wondering if I should set the tires into the calculator at 30" instead of 31" since they do measure 30" tall after being mounted and inflated on the wheel??

 

What is pointless about 3.55 to 3.73 and what about 4.10? I see you have 4.56 gears but pushing 35s not 31s so, could you explain what would be beneficial about 4.56 gears pushing 31s?

 

4.10s at least. 4.56? Meh.

 

This right here says everything you need to know. Go with the deeper gears.

IMO you're overthinking the RPM. Wear on an engine is not directly related to RPM. You must also account for the higher RPM producing more power thus straining the engine less at the same load. These engines have a short stroke and have no problem running higher RPM for extended periods of time.

I've also experimented with varying the tire diameter from the nominal size when using that calculator to try to account for size discrepancy. I tend to just use the nominal size though because its close enough. You gotta figure the OEM size is also likely not true to size, so they're probably comparable.

 

I have another XJ on 35s for playing off road and a 4-banger beater-with-a-heater, making my XJ on 31s my fun street vehicle. I pull my boat and utility trailers with this XJ, go camping with this XJ, do mundane things like shopping with this XJ, go ripping around having a bit of fun with this XJ, rescue dumb family members who decide to go camping before a 36" snow storm with this XJ. A vast majority of my driving is around 50 mph on country roads.

I'd be more than happy to do those speed tests for ya, but my XJ is lacking a steering wheel, brake and gas pedal at the moment, so, ... I also haven't yet corrected the speedometer, so it would only be a guess. The calculators should get it darn close, but remember that a 31" tire very rarely ever is 31". Some 31s I was looking at just the other day only measure 30.7", when brand new. Mine have a couple years worth of wear on them.

Change in rpm, and the increased power that comes into play at higher rpm, really isn't the story here. The story is the combined ratio of the components between the engine and the road. The AW4 has a 2.20 first gear, 1.53 second, 1:1 third and 0.75 fourth gear. So the torque of the engine gets multiplied by 2.20 or 1.53 or 1 or .75, then multiplied by the transfer case (2.73 in 4-LO, 1:1 otherwise), then multiplied by the axle, 3.55 or 3.73 or 4.10, and then divided by the radius of the tire. The math results in pounds-of-force and this is what causes the vehicle to accelerate.

Since it's all multiplication/division and we're only looking at the axle gear ratio, we can use simply percentages to see what the difference in performance is going to be. The difference between 3.55s and 3.73s is going to be 3.73 / 3.55 = 1.05, 5%. All that work for 5% improvement. Roughly equivalent to adding 9 hp / 12 ft-lb to the motor.

3.55s vs 4.10s is 4.10 / 3.55 = 1.15, 15%. Now we're getting somewhere... The equivalent of 30 hp and 35 ft-lb of torque from the motor.

3.55 vs 4.56 is 4.56 / 3.55 = 1.28, 28% Equivalent to 55 hp and 65 ft-lb.

I didn't calculate the numbers, but order-of-magnitude here, we're talking about an 100-200 rpm increase going from 3.55s to 3.73s and 200-400rpm going from 3.55s to 4.10s. In city driving, those numbers are meaningless because you're not driving at speed for any length of time, you're accelerating between speeds more often than not. When accelerating, your driving the throttle, not the speedometer. It's in this realm that power is king. Swapping in a larger motor generally results in less fuel usage if driven exactly the same. The exactly qualifier there is key. More motor, or more gear ratio, will take less fuel to achieve the same final force to the road, the same acceleration. That's a long way to say, if you don't have to press the pedal so hard on your 4.0, you'll save gas even with the higher RPMs.

On the highway, when you're just turning RPMs, your fuel economy will drop a bit. The math works the same, just in reverse. With 3.73s, you get 5% more power, but you also turn 5% higher rpms at any given speed. 15% for 4.10s, 28% for 4.56s. It shouldn't be too far off to use those numbers directly against fuel economy, so assuming you're getting the 20 mpg the XJ is rated for on the highway, you'd drop to 19 mpg with 3.73s, 17 mpg with 4.10s or 14.4 with 4.56s. Add in the increased rolling resistance of the bigger tires, and the increased air resistance from the vehicle sitting higher, and actual MPGs will likely be lower. The price we pay for living the dream...

IMO, 3.73s aren't worth the time and effort. The 5% power improvement is not going to make enough of a difference to solve your power issue.

 

I too run 31's on the standard 3.55 rear and aw4...the easiest and cheapest mod is to downshift from OD to 3 when climbing hills. It puts the gearing back to stockish and restores engine performance on the hills. If your running cruise control, it doesn't kick it off and when you crest the hill you just push it back into OD

 

I appreciate your time and effort.

 

I think Im slowly being convinced of 4.10s...

 

My transmission automatically downshifted before going up a hill and it dawned on me "Oh it was struggling to go up.." So, yes, Ive learned to do the same you have mentioned and downshift to direct drive when necessary. Thanks for posting that good-to-know trick.

 

Yes but when it downshifts on its own it almost never locks and the rpm skyrockets...if you downshift the Tranny will relock in 3 restoring efficiency