Performance 40s vs 37s

Basscat

Basscat

Member
Hope everyone had a wonderful Christmas!
I've seen lots of content on what's required to run 40" tires and 37" tires but nothing on the performance / capability differences between the two?

What are the trade offs going from 37" to 40"?

Doing 40s right (IMO) means full width axles and changes in back spacing along with other mods. Does the wider track width have down sides or is the extra 3" to 4" not noticeable?

I realize the extra unstrung weight and rolling mass are likely horsepower sapping but how much on a V8? Just barely noticeable and 5.13 to 5.38 fixes it or so much that horsepower mods are needed?

Coilovers would likely allow one to keep the same ride height (4") they had with springs and shocks on 37s? Yes? No?

Has anyone on here gone to 40s, not liked it and gone back to 37s? If so, why?

Finally, what's the performance / capability advantage of stepping up to 40s? I've wheeled on 37s and 60s front and rear with the old TJ and loved the unsparing weight. It was like the TJ was stuck to the rocks. JKUs are much bigger so my thinking is 40s are to a JKU what 37s were to a TJ.

Not talking cost here so don't need to waste time on what it all costs. Hell all jeeps cost money if you're into jeering. It's a way of life right!

I'm looking for honest input from those who have run both and can tell me what the differences in

1) off road performance and capability are
2) differences in on road driving manners are

Note: mine is not a daily driver but I don't want a borderline trailer queen either.

Thanks and happy holidays!


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Eddie had 40s on Rubicat for a while and then put 37s back on him. I think he was still on D44s at the time and Eddie had concerns around that much weight on such a small pinion. (Eddie, please correct me if I'm wrong there).

If you're doing aftermarket axles that allow for a larger pinion in a 5.38 gearing, then I don't see why not do 40s.

Hope others with more personal experience will chime in.

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Subscribed. I have the same questions.

Edit: not thinking of 40's anytime soon, lift/fender laws suck around here and I think 37's on a 2 door is plenty. Trying to decide between the prorock packages with a slim but possible V8 in the distant future.
 
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GraniteCrystal said:
Eddie had 40s on Rubicat for a while and then put 37s back on him. I think he was still on D44s at the time and Eddie had concerns around that much weight on such a small pinion. (Eddie, please correct me if I'm wrong there).

If you're doing aftermarket axles that allow for a larger pinion in a 5.38 gearing, then I don't see why not do 40s.

Hope others with more personal experience will chime in.

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Click to expand...
He ran 40/1550/r20s on rubicat to test dynatracs progrips brake kit. He still had the pr44 front and fullfloat pr60 rear geared for 37s. I believe his intention/interest was testing the braking performance with a heavy wheel and tire combo for Jim, and as a result running 40s on rubicat...

I remember seeing a post from Eddie a while ago saying something like-

he pretty much takes rubicat everywhere moby goes, it just takes a little more finesse...

-But don't quote me on that

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I'll play because I'm right there with you.

I agree with you, if you want to run 40's and do so comfortably IMO you need to run full width, full float axles. The extra width will help in many ways, keeping the cog low is key and keeping those tires out of the body. Full floating axles to keep that weight on the axle tubes and not just the flanges is just as important as well unless you don't mind swiping out unit bearings constantly and love trail breakdowns and repairs.

Reliability is the one thing I stress when building any of my vehicles, and I want them to be multi-purpose. I want to drive it to the trail/track and beat on it, then I want to drive it home. Then when home I want to get groceries or go to dinner or commute to work in the same vehicle. If that wasn't the case I would just build a stripped race car or a buggy in this case.

Performance.... This is just a word I've completely forgotten about with my Jeep. I have 37's now, 5.13's on a PR44/60 combo and while on the trail and in the rocks I cannot complain about anything. The minute I leave the trail and get on the freeway to head home I really start despising the Jeep.

So with you, I wonder if I need to change over to coilovers too if I went with 40's on full width axles or can I keep the 4" plush rides. I hope someone can help me with that because I love how the Jeep rides now, and really don't want to change it up.
 
tgoss said:
I'll play because I'm right there with you.

I agree with you, if you want to run 40's and do so comfortably IMO you need to run full width, full float axles. The extra width will help in many ways, keeping the cog low is key and keeping those tires out of the body. Full floating axles to keep that weight on the axle tubes and not just the flanges is just as important as well unless you don't mind swiping out unit bearings constantly and love trail breakdowns and repairs.

Reliability is the one thing I stress when building any of my vehicles, and I want them to be multi-purpose. I want to drive it to the trail/track and beat on it, then I want to drive it home. Then when home I want to get groceries or go to dinner or commute to work in the same vehicle. If that wasn't the case I would just build a stripped race car or a buggy in this case.

Performance.... This is just a word I've completely forgotten about with my Jeep. I have 37's now, 5.13's on a PR44/60 combo and while on the trail and in the rocks I cannot complain about anything. The minute I leave the trail and get on the freeway to head home I really start despising the Jeep.

So with you, I wonder if I need to change over to coilovers too if I went with 40's on full width axles or can I keep the 4" plush rides. I hope someone can help me with that because I love how the Jeep rides now, and really don't want to change it up.
Click to expand...
Why do you despise it on the road?

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tgoss said:
I'll play because I'm right there with you.

I agree with you, if you want to run 40's and do so comfortably IMO you need to run full width, full float axles. The extra width will help in many ways, keeping the cog low is key and keeping those tires out of the body. Full floating axles to keep that weight on the axle tubes and not just the flanges is just as important as well unless you don't mind swiping out unit bearings constantly and love trail breakdowns and repairs.

Reliability is the one thing I stress when building any of my vehicles, and I want them to be multi-purpose. I want to drive it to the trail/track and beat on it, then I want to drive it home. Then when home I want to get groceries or go to dinner or commute to work in the same vehicle. If that wasn't the case I would just build a stripped race car or a buggy in this case.

Performance.... This is just a word I've completely forgotten about with my Jeep. I have 37's now, 5.13's on a PR44/60 combo and while on the trail and in the rocks I cannot complain about anything. The minute I leave the trail and get on the freeway to head home I really start despising the Jeep.

So with you, I wonder if I need to change over to coilovers too if I went with 40's on full width axles or can I keep the 4" plush rides. I hope someone can help me with that because I love how the Jeep rides now, and really don't want to change it up.
Click to expand...
From what I have gathered, the only things the coilovers are going to do is give you are ride height adjustment, more travel, the ability to soak up high speed bumps better, and look cool as hell... so, if you find your current setup lacking while bombing through knee high whoops, or you seem to need a few more inches of flex on the rocks, then coilovers are the ticket. Otherwise, they are noisy, and require quite a bit more to setup and maintain. It has been said many times that the plush ride springs paired with king shocks ride "better" than the bolt on coilover kit, and it has also been said MANY times that if you THINK the DTD system is overkill for you and what you do with your jeep, then it is and you will most likely not utilize the system to its potential. While I have found a spot here or there I could use more flex on occasion, I am still overall happy with my spring/shock combo. I don't see any reason why you couldn't run a 40" tire, on full width axles, with the proper bump stop and a spring/shock combo.

Coilovers are not "necessary" to run a bigger tire.

With that being said, I have also yet to find myself on the trail thinking "I wish I had 40s" the only time that happens (to me anyway) is when I'm wheeling with a jeep on 40s... but the devil is in the details, because I don't have 40s, and yet we are both tackling the same obsticals...

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For arguments sake, the Curries seem to have done fairly well with 4" coils and shocks on 37s, 39s/40s

And for discussion sake, how would a coilover provide more travel if the shock had the same stroke length?

More adjustability, yes...coilover wins

I would think you'd be fine on your plush ride coils, make sure you have trimmed where necessary and have your bump stops set correctly

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I picked up my 37s with less than 1k miles as the guy went to ultimate 60s and 40s...he had that set up for less than 500 miles and regretted the decision, and said he wished he stayed on 37s

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I have 40" Mud Grapplers, 17" ATX Slabs, Dynatrac ProRock XD60 front/80 rear, and a bunch of armor, so my Jeep is heavy. It still has the 3.6/A580 power train. IMO it's a complete dog. It's fun around town, but here in mountainous northern Arizona, it's a bear to drive on the highway. Super chargers for the JK are all subpar IMO, so a V-8 swap is in the future for me. With that said, performance really is subjective. Everyone will have a different opinion on how fast or powerful something feels.


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DWiggles said:
From what I have gathered, the only things the coilovers are going to do is give you are ride height adjustment, more travel, the ability to soak up high speed bumps better, and look cool as hell... so, if you find your current setup lacking while bombing through knee high whoops, or you seem to need a few more inches of flex on the rocks, then coilovers are the ticket. Otherwise, they are noisy, and require quite a bit more to setup and maintain. It has been said many times that the plush ride springs paired with king shocks ride "better" than the bolt on coilover kit, and it has also been said MANY times that if you THINK the DTD system is overkill for you and what you do with your jeep, then it is and you will most likely not utilize the system to its potential. While I have found a spot here or there I could use more flex on occasion, I am still overall happy with my spring/shock combo. I don't see any reason why you couldn't run a 40" tire, on full width axles, with the proper bump stop and a spring/shock combo.

Coilovers are not "necessary" to run a bigger tire.

With that being said, I have also yet to find myself on the trail thinking "I wish I had 40s" the only time that happens (to me anyway) is when I'm wheeling with a jeep on 40s... but the devil is in the details, because I don't have 40s, and yet we are both tackling the same obsticals...

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Click to expand...

Yeah, from what I gather as well I agree with you on all the same points switching to coilovers. Weighed the pros and cons and I still like my set up currently. I have no problem bombing washes but obviously big whoop sections are a no go with king 2.5's and the plush rides, even with king bumps front and rear. I guess the only way to tell would be to just switch over though at that point there is kind of no return. However if my ultimate goal was to run the desert at high speed the JKU would not be my build platform. My main concern keeping this current shock/coil configuration on 40's is setting the bump at a point where up travel is very limited.

And to your last point, that's exactly it. Even when wheeling with Jeeps that have no lockers, on 35's and very basic suspension they are doing the same obstacles that we, locked on 37's are doing, they just have to work a little harder at it. Everyone gets through the trail together and that's what it's all about.

40's are beyond unnecessary for what I would ever need, but you hit the nail on the head.... the devil himself is in the details and damn they so good on a JKU or a stretched JK.
 
Robertcrav said:
For arguments sake, the Curries seem to have done fairly well with 4" coils and shocks on 37s, 39s/40s

And for discussion sake, how would a coilover provide more travel if the shock had the same stroke length?

More adjustability, yes...coilover wins

I would think you'd be fine on your plush ride coils, make sure you have trimmed where necessary and have your bump stops set correctly

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Click to expand...
Only because I generally do not like the idea of coil retainers, and if you run a 4" spring with a shock with the same stroke length as a coilover it will unseat. Just my $0.02

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Really appreciate all the input guys. I started this thread because sometimes I've had a tendency to go one step too far in "making my jeep better". Can think of a couple of times with past jeeps where the project cavitated and went the other way. I have the LS3. I'm just trying to figure out if 40s, XD60 / PR80, and coil overs, which I've never run kind of turn the jeep into a big lumbering and cumbersome poor handling rig with very little upside on capability / performance compared to the 37" (PR44 / XD 60 HP) with 4.5" dual rate spring and Bilstein 5160 remote set up. Anyway...... thanks and I sure don't mind if others who have been down this road chime in!


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rockwell said:
Subscribed. I have the same questions.

Edit: not thinking of 40's anytime soon, lift/fender laws suck around here and I think 37's on a 2 door is plenty. Trying to decide between the prorock packages with a slim but possible V8 in the distant future.
Click to expand...

That's a tough call. If 37s are the objective you don't necessarily need the pro rock package but you do need a PR 44 front to do it right IMO. The rear axle tube on the JKU is pretty beefy and new 4130s in a 32 spline (semi float) will certainly work, again just my opinion. The V8 is a game changer and with the right transmission (think 6L80/90) the old school thinking of chasing deep gears to compensate simply goes away!


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tgoss said:
I'll play because I'm right there with you.

I agree with you, if you want to run 40's and do so comfortably IMO you need to run full width, full float axles. The extra width will help in many ways, keeping the cog low is key and keeping those tires out of the body. Full floating axles to keep that weight on the axle tubes and not just the flanges is just as important as well unless you don't mind swiping out unit bearings constantly and love trail breakdowns and repairs.

Reliability is the one thing I stress when building any of my vehicles, and I want them to be multi-purpose. I want to drive it to the trail/track and beat on it, then I want to drive it home. Then when home I want to get groceries or go to dinner or commute to work in the same vehicle. If that wasn't the case I would just build a stripped race car or a buggy in this case.

Performance.... This is just a word I've completely forgotten about with my Jeep. I have 37's now, 5.13's on a PR44/60 combo and while on the trail and in the rocks I cannot complain about anything. The minute I leave the trail and get on the freeway to head home I really start despising the Jeep.

So with you, I wonder if I need to change over to coilovers too if I went with 40's on full width axles or can I keep the 4" plush rides. I hope someone can help me with that because I love how the Jeep rides now, and really don't want to change it up.
Click to expand...

I've got a friend with that exact set up. He's in a 2012 JKUR and has a herd time getting used to running at such high RPMs to get anywhere here in the mountains of Colorado. I keep telling him that Pentastar is meant to run in a higher RPM band but he's looking at an LS swap. The whole coilovers vs spring shock combo is where I'm hung up as well. I've seen some standard setups flex fairly well although obviously not as good as coilovers but pretty damn well!


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DWiggles said:
From what I have gathered, the only things the coilovers are going to do is give you are ride height adjustment, more travel, the ability to soak up high speed bumps better, and look cool as hell... so, if you find your current setup lacking while bombing through knee high whoops, or you seem to need a few more inches of flex on the rocks, then coilovers are the ticket. Otherwise, they are noisy, and require quite a bit more to setup and maintain. It has been said many times that the plush ride springs paired with king shocks ride "better" than the bolt on coilover kit, and it has also been said MANY times that if you THINK the DTD system is overkill for you and what you do with your jeep, then it is and you will most likely not utilize the system to its potential. While I have found a spot here or there I could use more flex on occasion, I am still overall happy with my spring/shock combo. I don't see any reason why you couldn't run a 40" tire, on full width axles, with the proper bump stop and a spring/shock combo.

Coilovers are not "necessary" to run a bigger tire.

With that being said, I have also yet to find myself on the trail thinking "I wish I had 40s" the only time that happens (to me anyway) is when I'm wheeling with a jeep on 40s... but the devil is in the details, because I don't have 40s, and yet we are both tackling the same obsticals...

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Click to expand...

Solid! Thanks. Really appreciate your input.


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Basscat said:
I've got a friend with that exact set up. He's in a 2012 JKUR and has a herd time getting used to running at such high RPMs to get anywhere here in the mountains of Colorado. I keep telling him that Pentastar is meant to run in a higher RPM band but he's looking at an LS swap. The whole coilovers vs spring shock combo is where I'm hung up as well. I've seen some standard setups flex fairly well although obviously not as good as coilovers but pretty damn well!


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Click to expand...

Which coilovers are you looking at?
 
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