Backspacing for 37s on PR60/80

J

JK57

New member
I have a 2014 Hemi JKU currently equipped with ATX slabs and 40x13.5 Nitto Trail Grapplers. Recently, I've been exploring scenic trails around Colorado with my family, but these mostly consist of dirt roads. Given the significant highway miles between these trails, I'm considering getting a second set of wheels and tires, which I'll call my 'economy set'. The axle width is 68.5 inches, and I want to optimize scrub radius as much as possible.


I'm not concerned about the appearance or stance since I can use my slabs when it matters. Additionally, my old Evo high steer system limits uptravel, so I won't adjust bumps when swapping wheel sets.


Here are my questions:


  1. With the axles geared at 5.38, is it correct to assume that I shouldn't go below a 37-inch tire?
  2. What amount of backspacing do you recommend for a 37x12.5 tire that would work realistically?
  3. Are there any affordable OEM options from the HD truck world that might be suitable? If I can scoop something up off marketplace for cheap that would be cool. But any recommendations would be greatly appreciated!
 
OverlanderJK said:
Maybe you need a stick Jeep for fire roads and not a Jeep with a hemi and 40’s.
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All joking aside, I'd have to agree with this ^^^

With the Jeep you have now and for what you're wanting it to do, a second Jeep that is stock would be a much better option than trying to get a second set of wheels that will NEVER give you what you're wanting.
 
I’m a one Jeep kind of guy. Different trails, different wheels and tires. Makes sense to me. Hell I throw snow tires on the highlander for different road conditions . This is not a novel concept.
 
JK57 said:
I’m a one Jeep kind of guy. Different trails, different wheels and tires. Makes sense to me. Hell I throw snow tires on the highlander for different road conditions . This is not a novel concept.
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Tires for snow is hardly the same thing as smaller set to be run with gears that have a very high ratio but hey, if it makes sense to you.
 
To answer your back spacing question and assuming you're running coil overs, 4.5" is about as much as I would recommend with your axles. If you're running standard coils and shocks, 5.5" will work.
 
Ah yeah the gear ratio. Maybe if I were commuting on flat roads I’d feel differently. It’s not a DD. I don’t think I’ll mind another 200rpm over mountain passes, and with all the other stop and go madness that is Colorado.
 
JK57 said:
Ah yeah the gear ratio. Maybe if I were commuting on flat roads I’d feel differently. It’s not a DD. I don’t think I’ll mind another 200rpm over mountain passes, and with all the other stop and go madness that is Colorado.
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LOL - I wasn't even thinking about flat road commuting. 5.38 is the right setup for 40's when running a 3.6L but being that you have a HEMI, you really should be running 4.88 gears, 5.13 at most. If you're not having overheating issues now, that extra 200 hp (and really, it'll be more in practice), keeping your engine cool over the mountain passes will be a legit issue or, at least from what I've seen.

If it were me, regearing to 4.88 is what I would recommend if you're really wanting to swap tires the way you're wanting to do. It'll help give you the best overall performance and without having to buy another Jeep. But hey, that's just me and really, what would I know.
 
It can get hot at times, esp because the nag1 can’t ever really find the right gear to be in at all times. If I manually bump the gear up while climbing it doesn’t do too bad temp wise.
 
JK57 said:
It can get hot at times, esp because the nag1 can’t ever really find the right gear to be in at all times. If I manually bump the gear up while climbing it doesn’t do too bad temp wise.
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Just like I knew you had overheating issues without you saying you had overheating issues, I can tell you that it will only get WORSE with a set of 37's and 5.38 gears. But hey, I'm not hear to try and convince you of anything - just to provide some real insight and help if I can. Clearly, you seem to have it all figured out as to what you're wanting to do and I think I've provided the best answers I could for your specific questions. Best of luck to you (y)
 
Im giving honest answers to get the best feedback. Re-gearing isn’t an issue. The Jeep was axled before it was hemi swapped by the previous owner so it makes sense that is geared for a 3.6.

Thing is, you said the Jeep can’t ever be what I want it to be. I’m not sure what you mean by that. If gears are all that’s in my way it’s not a problem.
 
JK57 said:
Im giving honest answers to get the best feedback. Re-gearing isn’t an issue. The Jeep was axled before it was hemi swapped by the previous owner so it makes sense that is geared for a 3.6.

Thing is, you said the Jeep can’t ever be what I want it to be. I’m not sure what you mean by that. If gears are all that’s in my way it’s not a problem.
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I meant in regards to what your Jeep is now. I later mentioned that regearing to 4.88s would help you to get there.
 
JK57 said:
Im giving honest answers to get the best feedback. Re-gearing isn’t an issue. The Jeep was axled before it was hemi swapped by the previous owner so it makes sense that is geared for a 3.6.

Thing is, you said the Jeep can’t ever be what I want it to be. I’m not sure what you mean by that. If gears are all that’s in my way it’s not a problem.
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Take your highlander on the fire roads. You’re driving a race car at the grocery store parking lot. Kind of dumb.
 
I’m not an elitist douche so I don’t mind driving my built jeep wherever I want to.

You’re giving the hemi way more credit than it deserves.
 
That PR-80 dif is going to be pretty close to the ground on 37’s.

I had installed new 37’s on my JK about 6-months before I bought my PR-60/80 set. I thought it would be ok to continue to run the 37’s until they wore out. 😳

Wrong! I upgraded to new 40’s and a fellow Jeeper got a great deal on some 37’s!
 
If you’re trying to improve on-road drivability and still run bigger mud tires, there’s a lot of info on this forum that can help you dial that in.

Not sure how much you’ve run through all that stuff yet.

I recently rebuilt my ball joints, replaced my tie rod ends, drag link ends, added a PSC big bore steering box and pump system, and balanced my tires, and 80 on the highway feels amazing again. And I’m running bigger beat-up mud terrains with 40% tread left.
 
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