So Ya Say You Want 37s?? Here Are Things to Think About....
 

We get folks pop up and start off with things like “I just bought this jeep, and I’m going to add a 4” lift and 37s……”. If that was or is you, don’t feel bad, this thread is for you! Just trying to have a nice easy place to direct people to rather than digging up an old thread with these comments.

Nobody wants to believe that moving from 35s to 37s should be that much more work. Slap them on, regear and roll on. Some of us will tell you it’s not that easy, or there are going to be unseen obstacles. This thread is not to discourage you from going with 37s, but just for you to really think through things that you may want, or rather need, to address. Your jeep isn’t going to blow up and become a brick by simply adding 37s, but all these little things add up to a big financial bundle at the end of the day, and although you may really like the look of 37s, the reality is the performance difference between 35s and 37s is marginal at best; however, the stress on all components with 35s is significantly less.

So here we go, I’m consolidating some information for you to ponder:

Axles – Let’s be honest, if you’re running a D30, nobody is going to suggest 37s because regearing a D30 for that is a waste of money. If you’re running rubicon axles you’re a bit better off, but the housing still has the same weaknesses. At the very least add C-gussets. Sometimes you’re just better off moving to an aftermarket axle and selling your factory axle. Some might be opting to add trusses as well with beefier TB brackets.

Axle Shafts – in the rear, the flanges on the factory shafts bend easily. You’ll be looking at some chromoly shafts. Even then, don’t think those flanges can’t bend if really hammered on. If you wheel it hard, you may be looking at either chromoly shafts or RCVs. Happy shopping.

Ball Joints - One of the biggest weaknesses of the D30/44 axles is the ball joints. There’s only so much the manufacturers can do with the space available. If you have factory ball joints still, don’t expect those to last more than a few potholes down the road. If you have upgraded to things like Synergy, Alloy, etc……well, best of luck to you, they’re a crap shoot (I burned through 2 sets of Synergies on 35s in a year). I can’t report on how well Teraflex BJs are holding up in general with 37”+ tires, but you’re likely looking at the higher end option or Rare Parts. There is another pricey option out there and I refuse to name them. You can search my user name and my rebuild thread of those units to see my experience…..which you may or may not agree with. If you are installing ball joints and don't yet have C-gussets, this is a good time to do them.

TB/TR/DL – If you have not upgraded all steering components, you’d better plan on it. Options and prices vary widely.

Drive Shaft(s) – If you don’t already have an aftermarket shaft, you’re likely going to be replacing the factory front shaft at the very least. This isn’t so much a “37” thing as it is just the suspension lift in general. On the front DS the boot protecting the CV joint on the TC side will rip and spew all the grease. The joint will dry out and eventually die. You cannot see the rip in the boot until you uninstall the DS, but you will see the tell-tale sign of a big line of grease under your tub right above that joint. On the 4dr the rear factory DS is usually ok for a while. I found, way back when, that if you flip the orientation of that DS you protect that spline boot a bit as it moves further way from the evap skid which is what typically tears that up when flexing. Still, you should probably budget for a rear shaft along the way as well.

Brakes – The factory brakes really need to be upgraded when you cross the 37 threshold. You could do this with various big rotor or big brake kits with corresponding costs associated.

Hydro Assist Steering – You’re likely moving closer to the edge of adding hydro assist steering. The cheap option being a Red Neck Ram from WTO for ~$650, a nice PSC system at double that cost, or a PSC big bore box for whatever those run these days.

Regearing – Most people aren’t going to be doing this on their own, and a basic 2-axle regear is typically $1500+/- (and higher in some parts). With the 3.6L you’re likely looking at either 4.88s or 5.13s. With the 3.8L, likely 5.13s or 5.38s. If you need or want lockers, you’re obviously adding additional funds here. Keep in mind that if you are adding lockers, you’re now likely adding new axle shafts with higher spline counts to match. For you Rubicon guys that have the factory e-locker, keep in mind that is not the most reliable locker out there and you're adding more stress to it. If that craps out, you'll be disheartened to realize your options are either replacing with yet another rubi locker, or upgrading to a new 35-spline unit which means buying new rear axles shafts again.

Wheels – You really need 3.5” backspaced wheels to run 37s. If you are running the typical 4.5” BS for your current 35s, you can use some adapters, but I’m telling you, you’re likely to be removing these tires more ofen to address small nagging issues here or there, and you’re going to get tired of removing these things each time you have to pull a brake rotor off for something. Also, running adapters is going to net you 3” BS which is even lower than you need. Every extra .5” is just moving that tire further out and adding more stress to the ends of the axle (C’s and BJs). Most of the 3.5” BS wheel options will lead you down the path of beadlocks, and that is going to have own obstacles. Options, and availability, are much more limited than your standard wheels. Also, most tire shops won’t touch beadlocks.

Control Arms – if you do not already have a full set of adjustable control arms, 37s is probably the point at which you are wanting some. The front is self-explanatory as they allow you to position that axle and adjust your caster. In the rear, the issue is that 37” tire really creeps towards the front of the wheel well. Even with a lot of pinch seam trimming, you could have marginal clearance, but visually that tire starts to look a bit funny IMO. Rear control arms are going to allow you to push that axle back again and rotate the pinion. This sounds like a piece of cake; however, it could open up a can of worms which could look like this:.

Exhaust – if you’ve yet to address this, the problem is the exhaust crossover from passenger to driver’s side creates clearance issues with your front DS when articulating. Some people will use exhaust spacers to scoot that back a couple inches. You can also re-route the exhaust with something like an AFE Y-pipe which will move the cross over from in front of the cross member to behind it. Be sure to protect your gas tank and fuel lines with heat shield. You can also get a muffler shop to re-route the same way. Often, a shop will do this for $50-$80 if you can explain to them exactly what you are wanting. If you have an auto tranny, this also helps with tranny fluid changes in the future. You’ll understand why when you try to drip that tranny fluid pan.

Bumpers & Pinch Seam - I've never had full width bumpers on a modified jeep, so it's easy for me to forget this one from time to time, but, if you are running a full width from bumper you're going to have clearance issues. You'll need to get comfortable trimming, flexing to check clearances, and possibly trimming more. You'll be trimming a good bit of pinch seam in the rear as well.


Obviously, your entire suspension needs to be set up properly so cost of parts (and labor if not doing yourself) comes in to play. You may also need to address fenders (flat fenders have a wide range of costs from free (cutting factory) to up over $1k). If you’re building this for offroad, you have a whole slew of additional optional expenses that include skid plates, diff covers, more axle armor, body armor for the jeep, programmers, quick disconnects, labor for welding things if you can’t weld. There’s a whole crap load of things that fall in to this pile of unknowns or discretionary spending.

If you already have a jeep you’ve been building and are rolling around on 35s, you may have already addressed a lot of this along the way. For people who just bought their jeep, it can be harder to wrap your head around the costs it takes to accomplish a complete build. There are so many variables, but you're easily talking $10k+ if starting from scratch. Everyone can take away from this thread what they need. It’s just food for thought, so you can apply some of it to your own build and budget if it applies.

I’m sure there are things I’ve left off here or there. Everyone is welcome to add things to the list if you think of something else. I’m not outlining the costs associated with everything above…..you can look things up when budgeting.

This is a good list, coming from someone who has 37s and has done axles, shafts, ball joints, TB/TR/DL, drive shafts, brakes, regearing, wheels, control arms and exhaust. I didn't do hydro steering but I did upgrade the steering box. Every word here is dead on accurate.

I would only add that when you do replace your ball joints, it would be a good time to reinforce your C's if you haven't already as it involves welding and heating up the area where the ball joints are.

Weird, I had that mentioned in there somewhere, but kept having formatting issues and maybe it got lost as it took me multiple tries to post it.

Figured this forum could use a few fresh stickies. We either keep directing people to old threads or keep saying the same things over and over. I got tired to typing it out or digging for that one old thread I listed things in previously. :thumbs:

Resharp001 this is awesome! You've been helping me with advice on my build on the other thread titled 37" tires and Synergy lift.
This looks extremely comprehensive and I'm going to keep popping in over here to see how the thread progresses.
Thanks again, Semper Fi.

There are some things that are not relevant to my build that I forget. I thought about bumpers this morning and how I've seen full width front's have to be trimmed a good bit for clearance. If there are other things people think of along the way feel free to throw them out and I'll update the original post so we can keep the list consolidated.

I'm also fine revising, re-wording, or correcting anything anyone disagrees with. I just feel we need one simple spot where people can find this information to save us all a bit of grief regurgitating the information.

another thing to consider are fenders. High clearance fenders may be necessary while adding another grand or more to the cost.

I would add the the new jeep owner wanting 37's to make sure to stay away form E rated tires. I would also look at the actual size and weight of the tire as they vary greatly between brands and models. Look at the section width of the tire not the advertised width. I have seen 20lbs differences between different 37" tires. Same thing with the wheels, you can buy 17" bead lock wheels that weigh as little as 27 lbs and as high as 52 lbs. It may not sound like much but it does all add up in terms of fitment and maintenance. As you get older those big heavy tires wear on your back too.

Don't go buy tires because a bimbo is pimping them on instagram, or you got a "smoking del you count pass up". Seems like everyone gets these smoking deals they have to build around. Buy a tire that is going to fit your wants and needs.

I'll add on here with this: when I first bought my jeep I knew nothing, and I was super poor, so I bought a stock 07 4 door JK with 4wd and absolutely nothing else. My first upgrade was a 4.5 longarm lift and 35 in tires. Without going step by step through successive upgrades I'll tell you this that those 35s tore my Dana 30 front axle to shit. I now run 40s on dana 60 F/R on my 3rd build with the same 07 JK.

Moral is that bigger tires look awesome but they really do test up your axles if they arent strong enough. And that's with out talking about speed and gas and RPM from gearing etc etc.

thanks for convincing me to stick with 35"s!!!