Sponsored

How sensitive is the Trac Lok to tire size difference?

NC_Overland

Well-Known Member
First Name
John
Joined
Feb 21, 2020
Threads
17
Messages
1,680
Reaction score
1,910
Location
Raleigh, NC
Vehicle(s)
2020 JT Overland
I was just thinking the other day that I should probably get a matching tire size for my spare. I have 285s vs my stock 255s and my spare is a 255. Would a Rubicon spare be close enough? 285/75/17 isn’t that much different from my 285/70/18s. Thoughts?

I wouldn’t care if I didn’t have a limited slip rear end. The ABLS or rubicon/mojave rear lockers wouldn’t matter.
Sponsored

 

azmojave

Well-Known Member
First Name
Bobby
Joined
Nov 2, 2021
Threads
48
Messages
2,498
Reaction score
5,186
Location
Phoenix, AZ
Vehicle(s)
2021 Sarge Green Mojave
Occupation
Retired Trucker. Current Sawyer
Rubicon is 285/70/17 so your tires are an inch bigger.
 

ShadowsPapa

Well-Known Member
First Name
Bill
Joined
Oct 12, 2019
Threads
178
Messages
29,091
Reaction score
34,572
Location
Runnells, Iowa
Vehicle(s)
'22 JTO, '23 JLU, '82 SX4, '73 P. Cardin Javelin
Occupation
Retired auto mechanic, frmr gov't ntwrk security admin
Vehicle Showcase
3
285/75/17 vs 285/70/18 is something like 2 revs per mile different.
No concern. A curving winding road is more difference.
 

azmojave

Well-Known Member
First Name
Bobby
Joined
Nov 2, 2021
Threads
48
Messages
2,498
Reaction score
5,186
Location
Phoenix, AZ
Vehicle(s)
2021 Sarge Green Mojave
Occupation
Retired Trucker. Current Sawyer
Ok. Thank you.

Now the question is, is that too much of a difference?
It’s hard to say. I think I would worry about it too much, but others might let it slide. I’m sure it’s not good long term.
 

Sponsored

OP
OP
NC_Overland

NC_Overland

Well-Known Member
First Name
John
Joined
Feb 21, 2020
Threads
17
Messages
1,680
Reaction score
1,910
Location
Raleigh, NC
Vehicle(s)
2020 JT Overland
It’s hard to say. I think I would worry about it too much, but others might let it slide. I’m sure it’s not good long term.
Ideally, a spare tire wouldn’t be on for long. I’m mainly curious if there is a published threshold somewhere. I do agree with ShadowsPapa that the type of road/drive probably makes a difference as well.

IIRC, my Canyon’s spare tire was a 16” wheel vs my 17” wheels, but the overall tire size came out almost exactly the same.
 

ShadowsPapa

Well-Known Member
First Name
Bill
Joined
Oct 12, 2019
Threads
178
Messages
29,091
Reaction score
34,572
Location
Runnells, Iowa
Vehicle(s)
'22 JTO, '23 JLU, '82 SX4, '73 P. Cardin Javelin
Occupation
Retired auto mechanic, frmr gov't ntwrk security admin
Vehicle Showcase
3
New comparison based on Bobby's statement of tire size (and he is of course right on the 1" diameter, but the thing I look for is difference in revolutions)

Roughly 18 revs per mile difference, I'd not worry over a short term. One can also use the spare on the front and move a front tire back so both rears are the same. It will pull, handle weird, but if you are concerned about the difference on LSD, it's a solution.
Driving mountain roads would likely give you that much difference on switchback roads.
LSDs usually "engage" or grip based on a larger speed or torque difference between axles. A slow difference of 18 revs/mile isn't horrible.

Jeep Gladiator How sensitive is the Trac Lok to tire size difference? 1651117717831
 

Blade1668

Well-Known Member
First Name
Darrell
Joined
Jan 7, 2020
Threads
11
Messages
2,344
Reaction score
2,119
Location
N. AL.
Vehicle(s)
90XJ, 91XJ, 91MJ, 05 LJ, 20 JT
Build Thread
Link
Vehicle Showcase
1
If it is a major concern for you, add a few extra PSI Air to the undersized tire and drop a few in oversized one short distance. That could be enough to offset the size difference. The additional air pressure vs larger tire could be figured out to make up tire size difference if tires are within a inch or so. Don't forget listed tire size isn't exact plus wear of tread.
"My W.A.G." I can say a 33x10.5r15 at 8 psi will pull hard to that side when other tires are at 30-35 psi. 🙄 especially if it's a front tire.
 

Rahkmalla

Well-Known Member
First Name
Jim
Joined
Oct 20, 2021
Threads
31
Messages
1,656
Reaction score
3,654
Location
NJ
Vehicle(s)
22 Gobi Manual Mojave
Build Thread
Link
If you're that highly concerned about a slightly undersized spare, make sure you always run it on a front wheel and don't engage 4A (and obviously no 4H or 4L, but that should go without saying). 18 revolutions per mile shouldn't matter to get you from the flat to the shop, but you can run it on the front for an added layer of security.
 
OP
OP
NC_Overland

NC_Overland

Well-Known Member
First Name
John
Joined
Feb 21, 2020
Threads
17
Messages
1,680
Reaction score
1,910
Location
Raleigh, NC
Vehicle(s)
2020 JT Overland
If you're that highly concerned about a slightly undersized spare, make sure you always run it on a front wheel and don't engage 4A (and obviously no 4H or 4L, but that should go without saying). 18 revolutions per mile shouldn't matter to get you from the flat to the shop, but you can run it on the front for an added layer of security.
Right, but my spare is still a 255, so definitely need to change it
 

Sponsored

Snake Eyes

Well-Known Member
Joined
Feb 10, 2022
Threads
11
Messages
659
Reaction score
1,088
Location
Las Vegas
Vehicle(s)
2022 Sport S Max Tow, 2011 Toyota FJ Cruiser
Old language: Most people have forgotten what used to be called a donut...the spare that was way smaller than your regular tires until they came up with the useless canister of gunk solution to get rid of spares. Many of those cars had limited slip.

Use the “donut” tire rule at of course your own risk….If the spare is smaller, due to difference in rotation speed, it should not be used more than 50 mph and not farther than is necessary to get a full size replacement (no more than 50 miles). Keep it in 2wd
 

ShadowsPapa

Well-Known Member
First Name
Bill
Joined
Oct 12, 2019
Threads
178
Messages
29,091
Reaction score
34,572
Location
Runnells, Iowa
Vehicle(s)
'22 JTO, '23 JLU, '82 SX4, '73 P. Cardin Javelin
Occupation
Retired auto mechanic, frmr gov't ntwrk security admin
Vehicle Showcase
3
Old language: donut tire. Many car spares are smaller size or used to be until they came up with the useless canister of gunk solution. Most of those cars had limited slip.
I wouldn't say most had limited slip. That was an option and not often ordered. It wasn't standard. I've had a number of those cars......... and still own two of them (4 if you count one I'm parting and one I'm trying to give away)

I'd actually bet less than 25% had LSD
In fact, there are those who track numbers and a Pontiac forum says less than 20% in 65, so it's doubtful there were even 25% sold with it in the mid 70s. You might hit 20-25% in the 80s but I strongly doubt it. No 1980s car I owned had it.

A quote from Pontiac people:
19.2 % if you are asking about 1965. Limited slip were under very few cars.

And they were talking about performance cars - the GTO, etc.


One of mine ->

Jeep Gladiator How sensitive is the Trac Lok to tire size difference? rt-tail
 
OP
OP
NC_Overland

NC_Overland

Well-Known Member
First Name
John
Joined
Feb 21, 2020
Threads
17
Messages
1,680
Reaction score
1,910
Location
Raleigh, NC
Vehicle(s)
2020 JT Overland
I wouldn't say most had limited slip. That was an option and not often ordered. It wasn't standard. I've had a number of those cars......... and still own two of them (4 if you count one I'm parting and one I'm trying to give away)

I'd actually bet less than 25% had LSD
In fact, there are those who track numbers and a Pontiac forum says less than 20% in 65, so it's doubtful there were even 25% sold with it in the mid 70s. You might hit 20-25% in the 80s but I strongly doubt it. No 1980s car I owned had it.

A quote from Pontiac people:
19.2 % if you are asking about 1965. Limited slip were under very few cars.

And they were talking about performance cars - the GTO, etc.


One of mine ->

Jeep Gladiator How sensitive is the Trac Lok to tire size difference? rt-tail
I’ll be ready when that question come up in trivia! 🤣
 

LostWoods

Well-Known Member
Joined
Mar 13, 2020
Threads
15
Messages
1,921
Reaction score
2,171
Location
Gilbert, AZ
Vehicle(s)
2024 4Runner / 1995 YJ
A Trac Loc is engaged 100% of the time you are moving under engine power so you'll always have wear. Just turning under throttle wears the clutches which is why they're generally undesirable - they wear out and most models these days don't have parts available to rebuild even though you technically could.

Essentially you have a clutch pack with teeth on the inside attached to the differential side gears which are connected to the axle shafts. The metal components of the pack have a tab keyed to a slot in the carrier housing. When the worm gears have torque applied from the engine, they apply an outward force on the side gears which engages the clutches and "locks" the carrier to the axle shafts via the side gears.

But in a daily driver, clutches are not all that strong (otherwise you risk the lunchbox locker in the rain/snow situation) so there's slippage and slippage causes wear. Different tire sizes simulate turning so you're basically in a constant state of wear whereas going straight normally causes none. Greater the difference in rev/mile, the greater the wear and heat generation so use should be minimal.

I wouldn't hesitate to run a tire an inch smaller but I'm not sure I'd go more than that. @Blade1668 has it right that you can just overinflate if you need to get a little more diameter out of the spare to match. Just keep it at max sidewall PSI all the time and I'm sure you'd be fine.
Sponsored

 
 



Top