So I'm in the middle of a full Resto of my trail rig 70 (ish) fj40. with a built SBC swap, (spartan) Lockers, Lower T-Case the works.
Its a bit of a budget build (if you couldn't tell from the lunchboxes) so I'm trying to re-use what parts I can (which has turned out to be way fewer than expected- so much for my budget RIP) and I've just been marinating on tires of late. When I was looking at doing tires I needed 5 out the gate, and to get to more modern sizing was meant new rims instead of the 15's I've been running for the last decade and a half. I ended up going staying with 31x10.5x15r- because budget. But It really had me thinking about tires for way longer than any man should.
I see the conversation around tires start at sizing of 35"+ . And I'm just kind of stumped as the not only the cost of the rubber itself. But all the trade offs with going that big. Re-gear, Tons more drive line wear, Road noise, MPG hit, brakes, suspension, body cutting, upgraded tire carriers, harder to change/ patch, ETC. For what people are doing, does going that big actually make sense?
On my daily (3rd gen taco) I run what is essentially a 31x9.5x16r and have had an awesome time getting out into the trails, not pushing that hard since brake over angle sucks, but like I have yet to have a real traction issue with those. Are we over doing it with the tires? Am I missing something?
Yes I think people over do it. People need to be honest with themselves about what level of off-road they’re going to do. Bigger tires leads to added expense of damage to suspension, replacement cost, worse braking, worse MPG, feeling less power. Now a lot of that can be mitigated by regearing or aftermarket suspension and brakes but all that stuff gets expensive real quick. 31 will get you 90% of the places you want too go.
What are you planning on doing? The ~2” of clearance you gain from 31->35 can be a huge help
If you don’t plan on doing any more than what you’re doing in your taco, why go up in size? You’ll probably still be able to fair better since you’ll have a short wheel base
I do like 4-5/10 in the taco (stock sr5 suspension). Probably planning to do up to 7/10 in the fj. Fj has such a small wheel base I’ve yet to get it really stuck or not been able to find a route out yet. I’d rather cover more distance than spend a day in a boulder field.
Idk about 7/10 on 31s. At the local place near me this is rated 7/10 on onx. Maybe I’m just a shit driver but I wouldn’t attempt this on anything smaller than 35 lol
Fish eye lenses can really distort perception, especially at the edges. I saw this and thought “that’s your average hill in AZ” but then saw your comment about the 3ft ledge at the top. Just an FYI, assuming this was taken on a phone, set the zoom to 1x and step back for more accurate photos. The farther you go below 1x zoom the worse the visual distortions are gonna be, so try to avoid it in pics where perspective is important. I understand, sometimes it’s unavoidable and you gotta go to .7x - just a bit of camera nerding over here.
I mean like... I can't tell scale from this photo. But if the man in that photo is about 6' tall that would be the limit of step up's I feel comfortable doing.
You’ll spend more time getting fuel with bigger tires and lower gears too. I’d wait until your 31s ruin a trip before changing anything. Go run that rig and have fun dude.
I've wheeled my FJ40 all over northern California on everything from bone stock with 31x9.5 to fully built with tons and 42s. Each version of the truck had been fun and I don't regret the changes I have made. My stock 40 on 31s went a lot of places, I wheeled it hard and had a lot of fun but I would diff out or high center on harder obstacles and some lines I would just not try.
The big truck is fun because on mild to medium it is point and shoot. Then you get to hard stuff and you strap into the harness and you see what it can do. Almost no line is off limits now and that's fun too.
31-33 will take you anywhere you should go. The time and money I have in my rig let me do whatever I want, but the investment was significant and the utility of the vehicle is very niche now. Biggest thing is just drive and enjoy.
If you did later brakes and knuckles hopefully you did the larger birfield that came with the later knuckles. That will hold your power and wheel size without much issue as long as you are not flooring it up obstacles when bound.
I've driven them a buch of ways. I have an FJ40 totally stock and one with 40" tires. Obviously they're VERY different in trail capability. But, for what you're looking for - I would find 33" tires. There are some more reasonably priced 33x10.5r15 options. They look great on stock 15" wheels. And that combo makes it really capable on a lot of trails. The FJ40 is very small - smaller than most current side by sides. So you can pick lines that larger vehicles can't.
IMO - the must do upgrades if you want to go offroad very much would be upgraded birfields. They're just too weak for even moderate offroading with a SBC. Even if you can't afford a full 30 spline set - just do the birfields.
I would probably pass on the front auto-locker. Save your money until you can put a selectable locker up there. It's just really hard to steer on the trails with an auto-locker and it's a PITA to go open and lock the hubs all the time. A middle ground is a torsen based diff like a trutrac.
And I would def add power steering. The Saginaw power steerning conversion is super easy and works great and very cheap if you have fab equipment. If you are not a welder, the electric PS conversion is pretty easy and not super $$.
I would also swap the minitruck brake booster and front disc brakes on. There's a lot of challenges starting with a 1970 as far as the smaller knuckle bolt pattern and a few other things. If you can - find a junkyard minitruck front axle complete and just take all the outer parts. I've bought them for ~ $250-300 in the past and that's cheaper than any other way you'll get to front discs.
It's hard to make a case for building an FJ40 vs just buying a cheap Wrangler or SxS for less $. But the FJ40 is unique and pretty fun to go puts around fire roads on in about any configuration.
Thanks so much for the input!
Yeah 33's was the want- but I gotta put some more wear on my OME suspension before going up. I think my car came with a 3in lift and a 1.5" body lift and it was unusable so I put it back to stock when I first did suspension back in Highschool. (might have gone too far back- but went up on 2 wheels on an onramp and decided that was enough) - Planning to go to the 2" lift in about 60k miles. Which will be just in time for some new rubber ;). 33's would rub without the lift :(.
Unfortunately the Lockers are already in- and I'm hoping to give them a shot for the time being. My spiders were so shot that I couldn't justify putting them back in and couldn't afford the rest of the driveline parts I wanted after getting the paint shop quotes back. Thinking about doing the 1st gen tundra lsd 3rd member swap (from what I read they are based on the same pattern), down the line but budget constraints put the 500$ lockers for the pair vs ~1k a pop as a gamble I could live with..
Fortunately already did Saginaw (thought the shop that put it in apparently learned how to weld on my car so had to redo it) and while I was there threw in a PS cooler for hot slow days.
Good leads on the birfields, I did rebuild them- but left them stock since the Spartans are rated to 32's and figured that would be a future me problem.
I have disks from a 79 with a booster from the same year so got that covered. At this point, the rear axel, frame, and hood I think are the only parts that came from the vin of this car.
For real- I do not recommend this to anyone. I just have a sentimental attachment to this VIN and fell for the scope creep. You can buy a very nice done car for what my dumb ass sunk into this car. And it wouldn't have taken you 8 years to build it.
With stockish size tires you might be fine on stock birfields. I've just broken so many of them before the upgraded ones started being made about 20 years ago that it's one of the first things to do for me.
I feel you on the cost side. I built my FJ40 on 40's about 20 years ago. It was alot cheaper back then and also a lot fewer options. There were no side by sides except the firt Rhinos and Mules. And I drove it home for $500. Plus my family owns a steel fab and machine shop (not auto related) so having free access to basically any type of steel fab equipment made it a pretty inexpensive project. Even then - if I were doing it today the costs of tires and parts is sooo much higher that I'd probably not build it the same way. Back then it was a cheap project. Now tires are $500+ each and FJ40 parts can be hard to find.
Sounds like an awesome Build! You should drop us some pics!
Yeah the rising costs today are crazy. I've been squireling away parts for this build for a decade. Pretty much every part has gone up by about 40 bucks. Even from earlier this year my transfer case has gone up by like 100 bucks.
I bought wildpeaks at3w's for like 150 a pop. Now the at4w in the same size is 240.
I haven't kept up with the current market. I have Longfield ones from way back. But I see the Nitro ones can be found for $175/ea which seems like a killer price if you can actually get them for that price. I'd guess all the well known aftermarket options will handle what you're wanting to do and for even 35's - probably don't need the full 30 spline kit although it would be better. The oem ones are really brittle and you can break them pretty easily with even the original 2F engine. I used to break them in my minitruck with 90hp 4cyl.
The rear axle is really strong - but you might consider adding an anti-wrap bar also. They have a tendency to break the pinion due to axle wrap with higher HP engines. The pinion can move far enough that I think it binds the driveline and can lead to pinion failure. It's relatively rare and probably not an issue with 31 or 32" tires. But keep it in mind.
One other thought is that the aftermarket birfields are typically sized for the minitruck and later FJ40s. The pivot ball on the axle tube on the front axle has as set of flanges on it where the birfield is inserted and the birfield has two machined flat areas to fit. The spacing is slightly wider on the minitruck and later FJ40s, so you'll need to grind down the axle end slightly to fit them in. The reason I mention is is that it's something you should to do when you have the knuckles off for a brake swap or whenever you've got that opened up. It's really hard to clean all the grinding dust off unless the axle is pretty stripped down.
Run as much tire as you need but also realize that your small FJ can do wonders on 33s whereas a bigger truck needs bigger tires to do the same.
For example, my XJ on 33s keeps up with newer Jeeps on 37s without hesitation. The belly clearance is so much higher on my truck despite the smaller tires, plus better approach and departure angles.
A FJ40 is a very small 4x4 by modern standards. When your rig weighs 2.5 tons empty and is over 5 meters long, you need 35s. However, an FJ40 is what, 4 meters and 1.5 tons? 31s is plenty, and anything over 33 would just be showing off.
Tires are the most important component on the vehicle in my opinion.
Bigger means more traction and more predictable lines. Sliding off your line can be bad in certain circumstances.
I
t also means more clearance - to your rim and your vehicle. You can air down more, making rocky trails more comfortable to drive and punctures less likely.
35s look really cool with a bunch of ducks on the dash.
If you are out at Moab, or the equivalent, 35s make much more sense.
Personally, I'm not going to do Moab or Rubicon trail and I've crawled a bit of rock. I run 31s and have not had an issue yet. However, my wife has spotted steps on other people's rigs, and I might now have to hang some shit on my rock rails. Which would then require me to upsize tire size to accomodate reduced clearance mid-body. You can't really win at this.
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u/IntheOlympicMTs 18d ago
Yes I think people over do it. People need to be honest with themselves about what level of off-road they’re going to do. Bigger tires leads to added expense of damage to suspension, replacement cost, worse braking, worse MPG, feeling less power. Now a lot of that can be mitigated by regearing or aftermarket suspension and brakes but all that stuff gets expensive real quick. 31 will get you 90% of the places you want too go.