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Discussion Starter · #1 ·
Hi All,

On my '16 Canyon AT, my spare is 265/70/16 versus 255/65/17.....so approx 1/2" taller

I assume using the spare on the rear would not wreak havoc with the rear differential as the spare would be spinning faster than the 17" tire?? I realize a spare is temporary.......but what if you are on a roadtrip and have to drive a long distance to get home??

I assume GM engineers took all of this into consideration and the small variable would be fine??

I will be installing 265/70/17 BFG KOs this weekend....now the variable gets much bigger, so I assume I should do something with the spare???

Thank you

George
 

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Take one of the front tires and put that in the rear, then spare goes in the front. Then 2wd all the way home, regardless of the weather conditions. I honestly don't know of the LSD in our rears have clutch packs or not, but if they do, what I suggested will be the only way to preserve them.
 
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Quick summary, showing circumference since that is the critical number (Pie x diameter, so the circumference difference is more than 3 times the diameter difference.)

Spare::::::::> 265/70/16 - 96.15 inches
Current::::::> 255/65/17 - 94.4 inches
Proposed::::> 265/70/17 - 99.29 inches

That seems like a lot to me, I think I would prefer to see the spare on the front (2WD, or at least in 2WD mode.) There are others here with more experience to give their opinions.

EDIT: Not sure I want to take advice from a @weasel1, but sounds like he agrees with me.>:)
 
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My truck being the 4cyl came with a donut I ended up buying z71 wheels and bought 5 of them so I have all wheels the same and all tires are the same size. If I was in you situation and that worried about it put the spare on the front and the front tire on the rear if it was a rear blow out.
Lol seems we all replied at the same time
 
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Just make sure if you ever have to run the spare, the rear diff has the same size tires. Put the smaller tire on the front, and keep the truck in 2wd until you can replace the bad tire.
^^^Ideally this

If you have to run it on the same axle for an emergency you should be fine as long as your not on pavement and keep it slow. As long as there is slippage in the tires (off road, loose surface) if the locking diff does occasionally lock it wont be too bad.
 

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Hi All,

On my '16 Canyon AT, my spare is 265/70/16 versus 255/65/17.....so approx 1/2" taller

I assume using the spare on the rear would not wreak havoc with the rear differential as the spare would be spinning faster than the 17" tire?? I realize a spare is temporary.......but what if you are on a roadtrip and have to drive a long distance to get home??

I assume GM engineers took all of this into consideration and the small variable would be fine??

I will be installing 265/70/17 BFG KOs this weekend....now the variable gets much bigger, so I assume I should do something with the spare???

Thank you

George
The locking diff we have is much different than a LS type. It won't do anything until one tire spins at about 100 rpm more than the other. So I'm thinking it would take a huge tire size difference for that to happen, even on turns. Someone with much better math skills than me could probably tell us how much of a difference it would take. I'm thinking it would not be an issue at all.
 

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The locking diff we have is much different than a LS type. It won't do anything until one tire spins at about 100 rpm more than the other. So I'm thinking it would take a huge tire size difference for that to happen, even on turns. Someone with much better math skills than me could probably tell us how much of a difference it would take. I'm thinking it would not be an issue at all.
Agree, not a problem unless you engage the locker, and this will only happen at relatively slow speeds.
 

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The locking diff we have is much different than a LS type. It won't do anything until one tire spins at about 100 rpm more than the other. So I'm thinking it would take a huge tire size difference for that to happen, even on turns. Someone with much better math skills than me could probably tell us how much of a difference it would take. I'm thinking it would not be an issue at all.
My math skills are rusty, but here is some fun for you.

See this discount tire website, with tire size calculator.

https://www.discounttire.com/learn/...RhVM0lX0M25b-gshgf4FmaSCLPO6gCzsaArQ7EALw_wcB

I plugged in spare and the proposed tire, it calculated that the difference in revolutions per mile between the 2 tires was almost 21 rev/mile. Doing what may be some rough math, at 30 MPH, the RPM difference would half of 21. It is just an anomaly if I pick 60 MPH, that the 60 MILES per hour cancels out the 60 MINUTES in the conversion from hours to minutes, so that the difference in RPMs of the tires at 60 MPH is 21 RPMS.

Obviously, turns makes this a bit worse, but how many sharp turns will you make at 60 MPH? Another rough calculation, using the turning diameter of our trucks at 40 feet, it appears that the revolution of the outer tire will be approximately 2 revolutions more than the inner tire if you make a complete of 40 foot diameter.

I don't think you could easily reach the nominal 100 RPM difference to activate the locking differential, also taking into account that above 25-35 MPH, it deactivates.

DISCLAIMER: All of these numbers are rough, and you take your life and the mechanical life of your truck into your own hands when you attempt any of these modifications.
 

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The locking diff we have is much different than a LS type. It won't do anything until one tire spins at about 100 rpm more than the other. So I'm thinking it would take a huge tire size difference for that to happen, even on turns. Someone with much better math skills than me could probably tell us how much of a difference it would take. I'm thinking it would not be an issue at all.
Speed (RPM) wouldn't play into it, this would be a constant difference, unless I'm missing something. For revs per mile, divide the number of inches per mile (5280 x 12 = 63,360) by the circumference (calculated earlier, above).

Spare::::::::> 265/70/16 - 96.15 inches = 658.97 rev/mi
Current::::::> 255/65/17 - 94.4 inches = 671.18 rev/mi
Proposed::::> 265/70/17 - 99.29 inches = 638.13 rev/mi

This shows about 2% difference between spare/current, and around 3% difference between current/proposed... straight-line.

I was going to say the LSD lockup percentages I'm aware of typically start in the 25% range and go up, but now that I think about it, I believe that figure relates to power transferred to the slipping side, not the... I think it's called the activation point. Anyway, more numbers to fuel the fire.
 

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Discussion Starter · #11 ·
Thank you all for the info. While reading additional posts on here I found that some people changed out thier spare to 235/85/16 which is very close to 265/70/17.

That offers me a spare that even closer in size than GM gave me as well as a narrower spare that I hope I will see less of, as it hanging out back. Anything I buy has to be better than that Maxxis tire GM gave us.

I do plan on venturing offroad. Mavis has the Yokohama Geolander ATs in 235/85/16 at a reasonable price.

Much appreciated everyone

George
 

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Thank you all for the info. While reading additional posts on here I found that some people changed out thier spare to 235/85/16 which is very close to 265/70/17.

That offers me a spare that even closer in size than GM gave me as well as a narrower spare that I hope I will see less of, as it hanging out back. Anything I buy has to be better than that Maxxis tire GM gave us.

I do plan on venturing offroad. Mavis has the Yokohama Geolander ATs in 235/85/16 at a reasonable price.

Much appreciated everyone

George
You mean we all made sense? We have failed in our mission to totally confuse you, I am so disappointed.
 
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Discussion Starter · #13 ·
Although I have decided to change my spare to keep it as close as possible without buying a 5th wheel, it does appear that there is some generous leeway available to us.

Some additional technical information from Eaton's website is below. The differential was designed to engage at speeds under 20MPH. So at highway speeds it will never engage. If this is the case, I would have to agree with others that it would take a very large tire size difference to cause any mechanical problems.

From Eaton's Website:

MLocker (G80) Operation During normal driving conditions, the MLocker (G80) functions as a light-bias limited slip differential. When a low-traction situation occurs that causes a wheel speed difference greater than 100 RPM, a flyweight mechanism opens to engage a latching bracket. The stopped flyweight triggers a self-energizing clutch system, forcing a cam plate to ramp against a side gear. Cam plate ramping will continue to increase until both axles turn at the same speed (full lock), which prevents further wheel slip. When the need for improved traction is gone - unlocking occurs automatically and the differential resumes normal operation.




Also in the downloadable sales literature:


At speeds above 20 MPH, the latching bracket
swings away from the flyweight mechanism
and prevents lockup from occurring
 

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Yeah plenty of room to breathe, at 20 MPH those variances are only work out to about 6 RPM!
 

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Hi All,

On my '16 Canyon AT, my spare is 265/70/16 versus 255/65/17.....so approx 1/2" taller

I assume using the spare on the rear would not wreak havoc with the rear differential as the spare would be spinning faster than the 17" tire?? I realize a spare is temporary.......but what if you are on a roadtrip and have to drive a long distance to get home??

I assume GM engineers took all of this into consideration and the small variable would be fine??

I will be installing 265/70/17 BFG KOs this weekend....now the variable gets much bigger, so I assume I should do something with the spare???

Thank you

George
I guess the obvious question if these are the stock tire sizes on your truck, what does the owners manual state when running with the spare tire?
 
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