Spare tire size difference

[Icutmetl]

What is the allowable difference in tire size on a limited slip or locking axle? 

 

I have the factory 265/60R20 tires on my 2018 All Terrain HD with a 265/70R18 spare, which appears to be 0.3% larger in diameter- not even enough to mention.

 

I would like to get rid of the mall-terrain Goodyear SRA tires at some point in favor of 275/65R20 snowflake rated all terrain tires, but am wondering what size to purchase for the spare. The closest size that popped up on TireSize.com is a 285/75R18, which is 2.1% larger in diameter. 

 

I know now it is bad juju for a non-open differential to have different tires on either end (in the case of our G80 rear). What is the allowable difference?  I’ve upgraded tires on past vehicles and increased the size an inch or so without following suit with the spare, but I never had a tire failure that required me using it. I wouldn’t mind doing it the right way this time around. 

 

Should I look for a used 20” factory rim? 

 

Thanks for your guys’ input!

[newdude]

Being you are changing the diameter a good bit, another 20" and a 5th tire of that size makes the most sense.  The only issue I see would be if it fits back under the truck.  Might have to bed it?  That hoist might scratch up that nice 20" a bit too. 

[Icutmetl]

9 hours ago, newdude said:

Being you are changing the diameter a good bit, another 20" and a 5th tire of that size makes the most sense.  The only issue I see would be if it fits back under the truck.  Might have to bed it?  That hoist might scratch up that nice 20" a bit too. 

That’s what I was thinking. I suppose I could keep my eyes out for a used one. I don’t want to keep it in the bed. I tend to over think stuff...