Sponsored

Tire and Axle Dilema - 37's vs. lightweight 40's

OBJLU

Well-Known Member
First Name
Henry
Joined
Jul 23, 2018
Threads
32
Messages
642
Reaction score
446
Location
Los Angeles
Vehicle(s)
2018 JLU Rubicon
I'm looking at new tires and I've narrowed it down to two choices:

  • Mickey Thompson 37x12.5r17 Baja Boss - by all accounts an amazing tire, I run the Cooper STT PRO's and everyone I talk to says these are a step up. They weight 81 lbs.
  • Geolander G003 MT 40x13.5r17 - a LIGHTWEIGHT 40 at 87LB's . I used to run these on a different rig and liked them but the Baja Bosses are likely a better performing tire
Here's my axle setup, I have aftermarket driveshafts:

  • Rubicon Axles
    • Front is Trussed and Gussetted if I do the 40's ill truss the rear
    • RCV front Axle shaft
    • Dana/Spicer Cromoly rear shafts
    • Dynatrac Ball Joints
    • 5.13 gears
I built these to run 37's without concern, although I know anything can break. I THINK i can get away with a lightweight set of 40's and still be in good shape. The Yokohamas are only 6 lb's more than the Baja Bosses.

What do you all think?
Sponsored

 

blnewt

Well-Known Member
First Name
Brad
Joined
Oct 8, 2018
Threads
96
Messages
9,876
Reaction score
23,795
Location
New Mexico
Vehicle(s)
2019 Jeep JL V6 SportS, (Retired 74 CJ-5, 80 CJ-7)
Occupation
Just ask @cosine he knows!
OP
OP

OBJLU

Well-Known Member
First Name
Henry
Joined
Jul 23, 2018
Threads
32
Messages
642
Reaction score
446
Location
Los Angeles
Vehicle(s)
2018 JLU Rubicon
Have you considered 37x13.5x17s? pretty good list of contenders here~
https://simpletire.com/tire-sizes/37-13.50R17
BFG KM3s & Toyo Open Country MTs come to mind :)
Thanks, the STT pro's are practically 13.5 and looking to size up but I think I'm going to go with the Baja Bosses since those are true-to-size so closer to what a "38" would be.
 

Sponsored

Mx5red

Well-Known Member
First Name
Paul
Joined
May 6, 2020
Threads
23
Messages
1,338
Reaction score
1,847
Location
San Diego
Vehicle(s)
2020 JTR, 2020 JLUR
Build Thread
Link
I have no experience, being new to the Jeep world, but increasing diameter of tires (and wheels) puts a lot more forces on the components than just the "scale weight" of the tire itself. So it's not just that the bigger tire weighs 10# more, its the fact it's 10# more AND it's on a longer moment arm.. Like a longer wrench.
So even if they weighed the same, it would still put a lot more force on your components.
Increasing negative offset or pushing wheels out further also adds torque to some components in a similar concept.
Search Rotational Inertia and gyroscopic Precession for examples...

Having said that, sounds like you've made your axles pretty beefy already!
 
OP
OP

OBJLU

Well-Known Member
First Name
Henry
Joined
Jul 23, 2018
Threads
32
Messages
642
Reaction score
446
Location
Los Angeles
Vehicle(s)
2018 JLU Rubicon
I have no experience, being new to the Jeep world, but increasing diameter of tires (and wheels) puts a lot more forces on the components than just the "scale weight" of the tire itself. So it's not just that the bigger tire weighs 10# more, its the fact it's 10# more AND it's on a longer moment arm.. Like a longer wrench.
So even if they weighed the same, it would still put a lot more force on your components.
Increasing negative offset or pushing wheels out further also adds torque to some components in a similar concept.
Search Rotational Inertia and gyroscopic Precession for examples...

Having said that, sounds like you've made your axles pretty beefy already!
Yahhh agree 100 percent, if I did 40's I'd try and find the "smallest" and lightest and even explore lighter wheels.
 

Creeker

Well-Known Member
First Name
Jim
Joined
Nov 19, 2019
Threads
16
Messages
566
Reaction score
1,537
Location
Hickory, NC
Vehicle(s)
CJ-7, 2020 JLR
Build Thread
Link
Here is some info one some MileStar tires.
This data is from a variety of sources.
Its the best I found.


SizeTypeLoad RangeLoad RangeMax Load (lbs@psi)Approved Rim Width (in)Tread Depth (in)Tread Width (in)Section Width
(in)
Overall Diameter (in)Maximum psiWeight (lbs)Speed RatingPly Rating
37x12.50R17LTMud TerrainD1243525@501019/321012.536.85071.3QD/8PR
38x13.50R17LTMud TerrainC11929981119/321113.637.83682.4QC/6PR
40x13.50R17LTMud TerrainD1213197@501119/3213.639.85082.4QD/8PR
 

Sponsored

OP
OP

OBJLU

Well-Known Member
First Name
Henry
Joined
Jul 23, 2018
Threads
32
Messages
642
Reaction score
446
Location
Los Angeles
Vehicle(s)
2018 JLU Rubicon
Lol
I knew a girl that weighed 90 and she thought she was obese. I'll say it again, a lightweight 40" tire? LOL
lol well usually they’re 100 lbs plus so yah
 

wibornz

Well-Known Member
First Name
Ted
Joined
Aug 3, 2018
Threads
157
Messages
9,799
Reaction score
49,913
Location
lansing, Mi.
Vehicle(s)
JL Unlimited Rubicon
Occupation
Retired from Corrections....I have stories.
If you are just going to Lowes or Home Depot, it doesn't matter. If you think that going to 40s and actually wheeling the Jeep, you will find it does matter. I think that if you go to 40s and wheel the Jeep, you must upgrade all the steering components. If you wheel hard, maybe think of hydro assist. The larger diameter tire also will require more braking power. Brake updates would be warranted also.

I thought about going to 40s, but to do it right and have the Jeep be dependable for the type of Jeep travel and wheeling that I do, I was looking at approximately $20,000 to do it right.
 

ChattVol

Well-Known Member
Joined
Jun 29, 2018
Threads
109
Messages
2,453
Reaction score
2,734
Location
Tennessee
Vehicle(s)
JLU 4 door
If you are just going to Lowes or Home Depot, it doesn't matter. If you think that going to 40s and actually wheeling the Jeep, you will find it does matter. I think that if you go to 40s and wheel the Jeep, you must upgrade all the steering components. If you wheel hard, maybe think of hydro assist. The larger diameter tire also will require more braking power. Brake updates would be warranted also.

I thought about going to 40s, but to do it right and have the Jeep be dependable for the type of Jeep travel and wheeling that I do, I was looking at approximately $20,000 to do it right.
I'm convinced 37/12.50/17 is the sweet spot for wheelin/road trips/all around use in the JL Rubi. Going to a 40" tire only nets 1.5" more clearance...yet it costs exponentially more to support it properly. I can't justify it either and would rather learn to be happy with 37s. I would wager a competent driver with a properly setup JL on 37s can do the majority of the trails that jeeps on 40s run.
 
Last edited:
OP
OP

OBJLU

Well-Known Member
First Name
Henry
Joined
Jul 23, 2018
Threads
32
Messages
642
Reaction score
446
Location
Los Angeles
Vehicle(s)
2018 JLU Rubicon
If you are just going to Lowes or Home Depot, it doesn't matter. If you think that going to 40s and actually wheeling the Jeep, you will find it does matter. I think that if you go to 40s and wheel the Jeep, you must upgrade all the steering components. If you wheel hard, maybe think of hydro assist. The larger diameter tire also will require more braking power. Brake updates would be warranted also.

I thought about going to 40s, but to do it right and have the Jeep be dependable for the type of Jeep travel and wheeling that I do, I was looking at approximately $20,000 to do it right.
I wheel alot and I know people who wheel alot running 40's with my setup, one of them just finished a few of the trails out in the hammers in Johnson Valley,. no hydro. Is it ideal? No but job got done.

If I go to 40's I'm going to run those trails too but I also daily it so I've decided to go with the Baja bosses and when I inherit my wife's car for a daily I'll move up to ,40's and plan for tons.
 

Darkseid

Member
First Name
Mr. Darkseid
Joined
Nov 26, 2020
Threads
0
Messages
11
Reaction score
3
Location
In My Mind
Vehicle(s)
JLUR
I wheel alot and I know people who wheel alot running 40's with my setup, one of them just finished a few of the trails out in the hammers in Johnson Valley,. no hydro. Is it ideal? No but job got done.

If I go to 40's I'm going to run those trails too but I also daily it so I've decided to go with the Baja bosses and when I inherit my wife's car for a daily I'll move up to ,40's and plan for tons.
So how's the jeep with the baja boss 40s?
Sponsored

 
 



Top