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Thread: Performance 40s vs 37s

  1. #1
    Been Around the Block Basscat's Avatar
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    Performance 40s vs 37s

    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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  2. #2
    Addict GraniteCrystal's Avatar
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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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  3. #3
    Hooked
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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.
    Last edited by rockwell; 12-26-2016 at 04:13 PM.

  4. #4
    Old Timer DWiggles's Avatar
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    Quote Originally Posted by GraniteCrystal View Post
    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.

    Sent from my Pixel using WAYALIFE mobile app
    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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    Last edited by DWiggles; 12-26-2016 at 04:21 PM.

  5. #5
    Addict Napalm's Avatar
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    Quote Originally Posted by DWiggles View Post

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

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    Just quoted you on that.
    IMG_1440.JPG



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  6. #6
    Knows a Thing or Two tgoss's Avatar
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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.

  7. #7
    Addict GraniteCrystal's Avatar
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    Quote Originally Posted by tgoss View Post
    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.
    Why do you despise it on the road?

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  8. #8
    Knows a Thing or Two tgoss's Avatar
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    Quote Originally Posted by GraniteCrystal View Post
    Why do you despise it on the road?

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    Because driving a vehicle with the aerodynamic properties of an outhouse down the freeway with a hamster wheel powering it is not my idea of enjoyment.

  9. #9
    Old Timer DWiggles's Avatar
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    Quote Originally Posted by tgoss View Post
    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.
    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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    Last edited by DWiggles; 12-26-2016 at 07:38 PM.

  10. #10
    Been Around the Block Robertcrav's Avatar
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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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    2015 Jeep Wrangler Unlimited Sport S - Currie RJ44's, 37s - Click Here For Build Thread

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