FJ40 Drivetrain Options/ Final Drive Ratio For 37s? (4 Viewers)

[DirtSniffer]

I am in the middle of my 40 build and am considering a change in direction from a more mild build on 33s and going straight to 37s. I had another build planned for big tires, but having thought about this all winter I have decided yet another rig isn't worth all of the money, hours, storage, insurance, etc. just to go up a size or two in tires and would like to just commit to one rig I will be able to enjoy more rather than have one sit most of the year.

I am sticking with my 40 as far as I have taken it, but will be making some changes to axles, drivetrain, and body to get it where I want it, which is a capable wheeler I can drive moderate distance, potentially cross country for big trips out west, and won't be afraid to wheel hard. Part of the equation will be deciding on a drivetrain setup. I want adequate gearing to run 37s (Will not be going bigger) with the 2F and hopefully not feel the need for additional gearing down the road.

That being said, what would a good rock crawling final drive ratio be for running 37s and the ‘ol 2F?

I have listed a number of results calculated using the GrimmJeeper gear ratio calculator for a variety of trans/tcase and black box combinations and approximate cost of parts that I’m sure others with the same dilemma will find useful for comparison. Unfortunately the only doubler option currently being produced for the H42/H55f is the NWF black box. These are all assuming 37s and 4.88 diff gears. (with one bonus 4.10)

Current prices:
NWF BB - $3600
4:1 split case gears for 34mm idler gear split case -$1300
Orion kit-$1950
H55f-$2400

Prices are only considering the above listed parts assuming I already have an H42 and split case. There will obviously be other costs associated with rebuilding a used trans/tcase.

H42 and 4:1 orion ($1950)
Final Drive- 70:1
2500 rpm cruise speed: 56
3000 rpm cruise speed: 68

H42 and 4:1 split case (10% underdrive) ($1300)
Final Drive- 70:1
2500 rpm cruise speed: 51
3000 rpm cruise speed: 62

H42 SS, NWF BB and stock split case ($3600)
Final drive ratios:
L1- 40:1
L2 - 47:1
LL - 108:1
2500 rpm cruise speed: 56
3000 rpm cruise speed: 68

H42 SS, NWF BB and 4:1 split case(10% underdrive) ($4900)
Final drive ratios:
L1- 70:1
L2 - 52:1
LL - 189:1
2500 rpm cruise speed: 51
3000 rpm cruise speed: 62
----------------------------------------------------------------------------------------------------------
[4.10 gears]
H42 SS, NWF BB and 4:1 split case(10% underdrive) ($4900)
Final drive ratios:
L1- 58:1
L2 - 44:1
LL - 159:1
2500 rpm cruise speed: 61
3000 rpm cruise speed: 73
--------------------------------------------------------------------------------------------------------
H55 and 4:1 split case (10% underdrive) ($3700)
Final Drive- 94:1
2500 rpm cruise speed: 60
3000 rpm cruise speed: 72

H55 , NWF BB and 4:1 split case (10% underdrive) ($7300)
Final drive ratios:
L1- 94:1
L2 - 71:1
LL - 257:1
2500 rpm cruise speed: 60
3000 rpm cruise speed: 72

H55 , NWF BB and stock split case ($6000)
Final drive ratios:
L1- 54:1
L2 - 64:1
LL - 146:1
2500 rpm cruise speed: 66
3000 rpm cruise speed: 80

As you can see, I would be struggling on long highway drives with an H42 and 4.88s. Going to the H55 will make a big improvement, even with the 10% reduction of the split case gears eats into that extra gear. However, with 4:10s the H42, BB and 4:1 split case gears option gives a wide range of gear options and good highway speeds, all while sticking with stock 4.10 gears. The 10% underdrive in high range would be about the equivalent of running 4.56s in high range with 4.10s, which would probably be livable on the street with 37s.

I currently have an FJ60 H42 with extension and split case. Having this option now means I can finish out my build using this combo and later add an H55 and 4:1 split case gears and not have to mess with anything. The black box, as cool as it is, is an expensive option and don't think I could make it fit with the H55 without moving the rear axle back quite a bit more than I already have. I could also find a short shaft H42 and potentially fit a black box and go the 4.10 diff gear route.

I am leaning towards the H55 and 4:1 split case option. I would have a 94:1 final drive ratio, good highway speed, and don't need to worry about an oil cooling setup on a black box for highway driving. The question though, will this be low enough or will I feel the need for a doubler when I get into progressively more technical trails down the road? The H42 short shaft, Black box, and 4:1 split case gears option is a close second, if deeper gearing is worth it.


Thoughts?

PFA, on 285/70r17s and FJ60 2" lift springs at the moment.

 

[pjohnson]

Edited...
I ran 37s on a SOA FJ40 with an H41:4.70 Toybox: 3sp transfer case: 4.10 differential gears. In all my rock crawling on the Rubicon, I never really needed lower than 101:1 I would engage the Toybox, leave the transfer in high range and use 1st and 2nd on the trans.
Also, I drove 5 hours to the trailhead and it beat the crap out of me. Any longer distance and it would be on a trailer.

These are the two gears I used 95% of the time.
1st 5.29x4.70x4.10=101.9
2nd 2.84x4.70x4.10=54.72

 

[EWheeler]

I have an H42/rockbox/4spd tcase with 4.10's and 37's on one of my 40s, it really is a nice setup for gearing options. You are able to cruise flat land highways easily with the flow of traffic with the rubber over drive. The gear splits in the h42 is nice for driving, lots of people say if there was any complaint with the H55 is that 1st is a bit too low for most on road driving, and the RPM drop from 2nd to 3rd is too much. Big long hills with 37's and 4.10's you're driving in the slow lane with flashers on with the 2f, but who cares, you look cool doing it. The rockbox is 3.4 to 1 and that was typically all that was needed for most trail driving around Moab & Colorado. For smoother roads, the stock low in the factory t-case was really nice to have at around 2:1. I wouldn't want to be married to 4:1 in low gear for a multipurpose vehicle personally. I vote FJ60 H42 w/ no extension, BB and stock split case with 4:10 diffs. This will leave you with a semi-reasonable rear driveline length and I think this is your most economical option. I think you're pretty low on your estimated costs on the builds of the low geared split cases. You can go onto Cruiser Outfitters website and build your desired case and get pricing. I about fell out of my chair seeing the total costs reach almost $8k for an H55f and low geared split case, granted these are all new parts, but I was pretty blown away how fast the costs creep up as you add features. The machining and welding of the case alone was $900.

 

[archie73]

I’m only on 35’s, but I’ll chime in on the 4:1. I really like it for the (limited) wheeling I do. With a sm465 granny and the 4:1, I can get around at idle on the rocks. If I need more wheel speed, I’ve got 3 more gears. Also I still on 4.11 in the diff, so it’s highway friendly.
The black box is interesting but I see two drawbacks-
1) additional drivetrain length on a lifted (6”?) vehicle is going to equal a short driveshaft. I know it can/has be done.
2)highway use- I keep reading about people having to work to keep the box cool. Once again I know it can be done, just seems like one more thing.
To be fair I’m internet jockeyingg those opinions, but that’s what I’ve read. I yield to someone with real world experience.

 

[DirtSniffer]

I have an H42/rockbox/4spd tcase with 4.10's and 37's on one of my 40s, it really is a nice setup for gearing options. You are able to cruise flat land highways easily with the flow of traffic with the rubber over drive. The gear splits in the h42 is nice for driving, lots of people say if there was any complaint with the H55 is that 1st is a bit too low for most on road driving, and the RPM drop from 2nd to 3rd is too much. Big long hills with 37's and 4.10's you're driving in the slow lane with flashers on with the 2f, but who cares, you look cool doing it. The rockbox is 3.4 to 1 and that was typically all that was needed for most trail driving around Moab & Colorado. For smoother roads, the stock low in the factory t-case was really nice to have at around 2:1. I wouldn't want to be married to 4:1 in low gear for a multipurpose vehicle personally. I vote FJ60 H42 w/ no extension, BB and stock split case with 4:10 diffs. This will leave you with a semi-reasonable rear driveline length and I think this is you're most economical option. I think you're pretty low on your estimated costs on the builds of the low geared split cases. You can go onto Cruiser Outfitters website and build your desired case and get pricing. I about fell out of my chair seeing the total costs reach almost $8k for an H55f and low geared split case, granted these are all new parts, but I was pretty blown away how fast the costs creep up as you add features. The machining and welding of the case alone was $900.

Thanks for the input. If I went the H55 route, I would be buying the 4:1 gears and putting them in the split case I already have, doing the machining myself. Gears alone would be $1300, but there will be other parts needed for the full rebuild that I would need to do to it anyways. I guess one upside of the H42/BB/4:1 route is that the 10% high range reduction will help some with the 4.10s so not as bad. I live in flat MN, Colorado and the like would maybe be a once a year trip for this rig if that, as I am also building an 80 for the more overland-y out of state trips.

I have a line on a possible short shaft FJ60 H42 that I might try and snag once the snow melts here in a few weeks.

Plan would likely be to run the split case without parking brake drum, which should save a bit on overall length, and maybe run an 80 rear which is offset about 2" less than a 40 diff I believe, should still be fine? But I confirmed with NWF that you can run an HF1A from an FJ80 behind the black box, but not sure if anyone makes gears for that case.

Got some thinking to do, I would like to have the transfer case setup before doing floor pans, especially if I end up going BB as then I could clock the case up.

 

[CruisinGA]

I run 37’s or 38’s with 4.10 gears
Had an sm420, sucked on the road and rarely needed first gear in low range plus the 1st to 2nd gap was so bad.

Now run an H42 with 3:1 in transfer. Much better

 

[White Stripe]

With 37s you will need 60-65" wide axles or it will be too tippy. Final low range should be between 65:1 to 150:1 depending on how rough of trails your going on.

 

[pjohnson]

A low SOA with flat springs keeps the tippy to a minimum.

 

[DirtSniffer]

With 37s you will need 60-65" wide axles or it will be too tippy. Final low range should be between 65:1 to 150:1 depending on how rough of trails your going on.

40 axles would be sold for something wider. Would like a FF 80 rear. I have an FJ60 front housing I robbed for a disk conversion that I may see about stretching to match and maybe run six shooters. Or steal the knuckles back from the 40 axles. Would like to avoid running an 8" front for this kind of rig, so 80 front is off the table for now.

Got a quote for stock height FJ60 springs with orbit eyes from Alcan, little over $1700. Bit spendy. Also considering creeper joint shackle hangers and regular leaves. The Liquid Iron Industries slider box is an interesting option to replace shackles and get a nice low SOA. I know a number of people have run them but have not heard much feedback. All the used 60 springs around here are too rusty, my front axle came with a set and while hammering out the shackle pin the spring eye broke clean off.

 

[White Stripe]

40 axles would be sold for something wider. Would like a FF 80 rear. I have an FJ60 front housing I robbed for a disk conversion that I may see about stretching to match and maybe run six shooters. Or steal the knuckles back from the 40 axles. Would like to avoid running an 8" front for this kind of rig, so 80 front is off the table for now.

Got a quote for stock height FJ60 springs with orbit eyes from Alcan, little over $1700. Bit spendy. Also considering creeper joint shackle hangers and regular leaves. The Liquid Iron Industries slider box is an interesting option to replace shackles and get a nice low SOA. I know a number of people have run them but have not heard much feedback. All the used 60 springs around here are too rusty, my front axle came with a set and while hammering out the shackle pin the spring eye broke clean off.

You can widen each side of the stock 40 housing if you want as a inexpensive option. Take some thick Dom tube, cut two pieces 5" long. Machine an inch of the inside on one end so the knuckle ball will snugly fit, machine an inch on the outside of the other end so it will fit snug into the axle housing. Weld it up and now you got a fj40 housing that is 3" wider on each side. As far as springs, try to get reverse eyes whatever you do.

 

[DirtSniffer]

You can widen each side of the stock 40 housing if you want as a inexpensive option. Take some thick Dom tube, cut two pieces 5" long. Machine an inch of the inside on one end so the knuckle ball will snugly fit, machine an inch on the outside of the other end so it will fit snug into the axle housing. Weld it up and now you got a fj40 housing that is 3" wider on each side. As far as springs, try to get reverse eyes whatever you do.

An option I may concider. I would be doing that with the 60 axle anyways, and would love an excuse to get a lathe... One attraction to the 80 rear however is it is disk brakes and has parking brake provisions already. And being full float I feel trail repairs would be easier than dealing with the 40 c-clip axle.

Reverse eye springs, I had forgotten about that option. I will add to the list...

 

[White Stripe]

An option I may concider. I would be doing that with the 60 axle anyways, and would love an excuse to get a lathe... One attraction to the 80 rear however is it is disk brakes and has parking brake provisions already. And being full float I feel trail repairs would be easier than dealing with the 40 c-clip axle.

Reverse eye springs, I had forgotten about that option. I will add to the list...

I would only keep the widening as a front axle option. Too many good rear axle options available.

 

[EWheeler]

Would like to avoid running an 8" front for this kind of rig, so 80 front is off the table for now.

I would not at all be afraid of the 8" front diff/housing, using one may save you some headace/lots of work. Lots of these units live under heavy 80 series and surive just fine. You may run into a road block with the 80 series tie rod/drag link setup with the leaf springs and the HP diff. May also have oil pan interference issues with the HP 8" diff? But, that being said, I think it is worth a look at an 80 series front axle for the width/stability aspect....even if you swapped 40/60 series knuckle balls onto it to keep parts you already owned, just have to buy custom inners, which you already have to do if you're widening a 60 series axle or 40 series axle as discussed. The FJ80 housing is stronger physically than the 9.5" front housings due to it's formed shape. Something to consider.....

 

[DirtSniffer]

Not 100% opposed to the 80 front, would need hellfire knuckles. I guess R&P strength may not be as much of an issue on the lighter 40, and blowing a front won't leave me stranded if I add selectable hubs. I'll take a look at the front axle on my 80 when I get a chance and see how it compares. Hopefully I would be able to sell the 40 axles I already built if I went that route, plan on trying to sell a bunch of parts this spring to fund new part purchases.

 

[pjohnson]

It's easy to overthink it (read $$$$$).

I try to stay focused on my primary purpose for the vehicle. Rock crawling is almost a purpose built rig (btw, y'all got rocks in MN?). There are a few examples here of well built, not overbuilt, capable rigs. @White Stripe comes to mind.

The build threads provide good examples of well built and overbuilt.

 

[Firemedic831]

Front Range Offroad make knuckles for the 80 series Or better yet just have them make you a housing

 

[DirtSniffer]

It's easy to overthink it (read $$$$$).

I try to stay focused on my primary purpose for the vehicle. Rock crawling is almost a purpose built rig (btw, y'all got rocks in MN?). There are a few examples here of well built, not overbuilt, capable rigs. @White Stripe comes to mind.

The build threads provide good examples of well built and overbuilt.

We do get some decent rocks in a few spots, mostly up north, but the most fun will be had on out of state trips.

 

[DirtSniffer]

One more set of numbers if one were to run an HF1A and marlin 3.12:1 tcase gears. HF1A would add about 1/2 inch to driveline but be correct offset for an 80 rear. Gears are easy to install as well and a bit cheaper. Don't get high range underdrive however.

H42, NWF BB, HF1A w/ 3.12:1
Final drive ratios:
L1- 46:1
L2 - 40:1
LL - 124:1
2500 rpm cruise speed: 67
3000 rpm cruise speed: 81

On another note, I just went and measured my caster on the 40 axles as they sit, and looks like I only have about 1* of castor. Was hoping the centered 60 spring pins would rotate the axle back some, but I should have planned on a C&T anyways for the pinion, although the angle is fine. Shifting spring mount probably countered the 60 spring angle. Adding widening sleeves isn't a whole lot more work if doing a C&T, so I may end up doing that and keeping this front axle and modify.

Anyone know if lcwizzard still sells the 2 5/8 sleeves? Left him a PM.

All the salvage yards around here appear to pull the front axles and scrap the rears, so sourcing an 80 rear may take some time. Looks like I missed out on a good deal for a set next state over a month ago. Oh well.

 

[DangerNoodle]

I've never had a problem with my 80 front on my 40 on 37 while wheeling it relatively hard. I have 5.29s in the axle, but also 8 gears to play with for speed. I'd be very surprised if you can get a 2f to pull hard enough, even with gears, to 81 MPH with 37's. Flat ground, maybe? Hills will be rough lol.

 

[DirtSniffer]

I've never had a problem with my 80 front on my 40 on 37 while wheeling it relatively hard. I have 5.29s in the axle, but also 8 gears to play with. I'd be very surprised if you can get a 2f to pull hard enough, even with gears, to 81 MPH with 37's. Flat ground, maybe? Hills will be rough lol.

I would actually like to run an 80 front if I can, but I would need to sell my 40 axles first and I'm not sure I'll find someone willing to pay what I have into them for parts in a timely manner. But we'll see. And no, I don't expect nor would I want to do 81 in a 40 with a 2F on 37s... lol. Just comparing gearing.