Time to build my 60 series, looking for best route to run 37’s…

Mar 14, 2008
Doublers are complicated. Several options. T-case gears can be a substitute.

Recipe: 5.29 gears, 35" tires, H-55 trans swap, 4-1 t-case gears, body lift, shackle reversal up front(gives you about an inch more lift), 80 series rear axle, IFS hub swap front axle, RCV alloy shafts with 300m inners, chromo hub gears, ARP knuckle and hub studs with a 5th stud conversion on an 80 series based crossover steering, hydro-assist box and ram, selectable lockers, sliders, belly pan, rear quarter guards, bumpers, winch, dual battery setup, header, cam, head work, good exhaust, good carb or efi, more fuel capacity, better seats.

That ought to get you started.


Feb 9, 2021
Salt lake city
A lot of good advise up above, I will add some pointers. SOA with a repacking of leaf springs and a longer shackle you could fit 37s. I would second that the 42H and transfer case are a horrible combo for that size tire. Not due to strength of the transmission or transfer case but due to the limited gearing combos.
On my off-roader I choose the LQ4 engine 4L80e transmission and the NP241 transfer case as my starting point. Have that paired with SOA and a longer leaf pack and some reverse shackles. Had to ditch the toyota rear axle unfortunately. On my daily I also used a LQ4 engine but went with the H55F transmission and transfer case on 33s for drivability.
May 23, 2015
Kansas City
Thanks for all the great advice. I’ve landed on SOA with 37’s. Sticking with the stock axles for now, ARBs, 5.29’s plus a 4 to 1 gear set for the transfer case with a 10% under drive. It may scream on the highway a bit, but should pull a grade better this way. I also have a high steer kit on order and will look into longer anti-inversion shackles if I need the clearance. I’ll post up pictures when it’s all said and done. I just hope it does not get too tippy in the rocks!! Thanks for all the help with tried and true solutions. If or when it ever comes time for a motor swap. A 350 and one tons would be on my build list.

If I’m missing anything, please let me know. Thanks.

Old Nick

Jul 10, 2020
Bourbon Country
A few questions.

Have you run your gearing through the Grimmjeeper gear ratio calculator?

Have you driven a SOA 60 series?

Have you ever driven a 2F over the Rockies or the Sierra's?

How "in-love" are you with the idea of 37's?

Are you going to stick with Toyota axles forever?

Are you willing to have a doubler setup?

You sound experienced. Here's my 2 cents.

Without a doubler, that stupid H-42 transmission has a 39-1 crawl ratio. With 36" tall tires, (Most 37's are 35.5-36.4" tall when actually measured) your speed at 700 rpm in low range will be just shy of 2 mph or about 165 feet per minute with 4.88 gears. You are going to hate that on the Rubicon and similar trails. Reverse is 48-1. That gives you 1.5 mph in reverse at 700 rpm, or about 138 feet per minute. In a 6,000 lb rig. (My 62 weighs 5600 lbs with no armor and 36" swampers) you are asking too much of 132 horsepower at 3200 rpm and 200 lb/ft of torque 1600 rpm. You are going to burn that clutch up on the first good trail and hate life as you bang and clang across the rocks and beat the crap out of yourself and the rig.

On the highway, your cruise speed at 55 mph will be 3500 rpm in 3rd and 2500 rpm in 4th. That works until you try to drag 6K of rig up a 6-10% winding grade at 7-8K ft of elevation. Your going to be in 3rd gear at 3500 rpm a lot and you are going to suck as much gas as a big block chevy doing it. At 70 mph, which will feel quick in a SOA 60, you are at 4500 rpm in 3rd and 3200 rpm in 4th. If you are lucky and it's healthy, that 2f will pull you at 70 on the flats, but it will be on the floor or close to it.

With 37's, your rotating weight is heavy. Steel wheels will hurt your ability to accelerate, brake and pull hills enough that you'll notice when you swap to a set of aluminum wheels. Forget turning a set of 37" Toyo's that weigh 150lbs each mounted, look for something a little lighter unless you are o.k. with the tradeoffs. Stock brakes are going to suck. At a minimum, you'll want to upgrade to the later IFS style calipers and a good pad that has at least an FF rating so you can get enough cold pad grip to stop them. With a stick, you shouldn't be burning up the pads on downhills if you're willing to downshift and be patient. The rears are pretty good for drums.

Without sway bars, you'll miss your 100 series after the first week or so of mountain roads. With sway bars, you'll hate that the only way to get disconnects is to make them or swap to a Currie anti-rock style swaybar. You'll probably hate the factory sway bars that hang under the axle too, but the only way to get a leaf sprung 60 to flex much is with soft springs. Speaking of springs, your 2" lift springs will bring your SOA height to around 7-8" of actual lift once you do the conversion. They will also be positively arched, which helps with droop but increases your chance of inverting a shackle and bending a spring without some form of limiting device. You'll also need a traction bar of some sort, as the SOA exacerbates leaf sprung axle wrap.

Onto axles. Toy axles are limited to 5.29 gearing. That means a good crawl ratio requires a doubler or deep t-case gears, or both. Birfields are tough, under a mini truck on 37's. They aren't that tough under a 60 series with 37's, crappy gearing and abuse. We also have to discuss that rear semi-float axle. Just say no, to that right now. There are no more new aftermarket alloy shafts for the semi-float setups. You WILL break a short side shaft, right at the splines with 37's, a locker and wheelhop on a tractive surface. Digging that shaft out of an ARB rear locker may require a torch and ruin both the locker and the housing, not to mention, where are you going to find an unobtanium semi-float factory shaft on a 3-day weekend and a way to get it to the trail?

Before I make you quit the plan, go watch some youtube video's of flatfender jeeps on the Rubicon. They get it done, with small tires, armor and going slow but body damage will be the result in a wagon. They are also 1/2 the size and 1/2 the weight of a 60.. with better gearing.

Here's my recipe(s).

Lift: Lift it just enough to clear the tires. Remember, there are almost no available lift kits above the 2" range anymore without custom ordering one or finding someone that still makes them but doesn't advertise them. SOA works but has it's limitations. Stability is one of them. The Wagon Run on the Rubicon has a 60 that flops almost every year. Pretty sure, most all the flops are SOA versions.

Transmission: H-55 or H-41 swap with a granny gear low. Barring that, you can so an SM465 swap, if you can find the adapters. There are no Auto's that go behind a 2F unless you configure an FJ62 version and they really aren't worth the trouble.

T-Case: Crawl box or 4.0-1 transfercase gears, or both. Underdrive high range gears come with some of the 4-1 gearsets to help you cheat that next step.

Axles: 5.29 gears in the Yota axles, or swap to some Dana stuff and get 6.17's if you want to run the 37's. Otherwise, run a 35" tire, and gear to your liking. If you have to go 37's, then your are probably stuck with SOA. Just remember, you lose 3-4% horsepower for every 1,000 ft of elevation you gain. An engine, even a 2F, will run a lot longer at 3500 rpm in peak power at 3/4 throttle than it will at 3100 rpm lugging along on the floor and losing rpm every 1/4 mile as you climb grades.

Tires: Speaking of tires, lighter is good. Skinny works well too. A good set of 35's that actually measure 34" or so, will take you 90% of the way that 37's will take you. That extra 10% will cost you $$$. Try to keep a smaller wheel size so you can keep a good, flexy, sidewall. Airing down is the best way to make a leaf sprung rig ride like a coil sprung rig...

Engine: Add some HP and efficiency to that 2F. A good cam, mill the head to bring compression out of the 7-1 range, a header and a good carb that is jetted for power will do wonders compared to the stock motor. Otherwise ditch the stock motor. Yes, the LS platform is cool, but you don't have to, just because everyone else is doing it. A mild 350 cubic inch engine of any sort will do nicely. Remember, it's about torque at usable rpm, not peak power. A 383 with 300 hp at 4K rpm and 400 lb/ft at 2200 rpm will be much nicer than a 5.3 that peaks 330 lb/ft at 4000 rpm. If you want more, a 450+ cubic inch engine that loafs along at 1800 rpm with highway friendly axle gears will return close to the same mpg as a 2F that is screaming it's guts out. (Ask anybody that drives a school bus or a dump truck with a gas engine the difference in mpg between an old 6 cylinder and a big block... About the only difference is how long it takes you to get from one gas station to the next..)

Other factors: Don't discount wind resistance. You already know this, but leave that roof rack and rooftop tent at home. Wind resistance goes up exponentially as you go up in speed. They are also top-heavy. Fuel: You're going to need a lot of it at 10 mpg. The factory 20 gallon tank isn't enough once you are out west. Jerry cans work, but that long-range tank sure is easier than carrying 3 five gallon cans to have equivalent range.

Good luck, best wishes with your endeavor.
Just passing through. Read your write-up. Truly excellent advice.

Users who are viewing this thread

Top Bottom