Does the spare tire have to be the same size (height) as the tires on the truck when using the spare?

[BlueMax 23]

I had a flat on the drivers side, front tire this week-end. I pulled the spare and saw it was a 17 rather than a 20 inch tireI thought it would be okay and with some reservation I mounted it and headed home.  
 

About 15 miles into a 20 mile trip home in the dark I realized there was smoke boiling out from the undercarriage.  

 

Long story short, the differential is now toast and has to be rebuilt.  
 

The question is - is this because of using the smaller spare tire or was it just a coincidence? 

[madweazl]

The wheel size (17" or 20" as you referenced) isn't important (provided it clears the brake package). What is important is the tire size (specifically diameter). Our '24 has 275/65r20 (34.1") with a spare measuring 275/70/r18 (33.2") for a difference in diameter of roughly 3%. I wouldn't expect that to have a detrimental impact to the LSD but I'm sure the further it's driven the hotter it gets. 

Edited September 14, 2023 by madweazl

[newdude]

2 hours ago, BlueMax 23 said:

I had a flat on the drivers side, front tire this week-end. I pulled the spare and saw it was a 17 rather than a 20 inch tireI thought it would be okay and with some reservation I mounted it and headed home.  
 

About 15 miles into a 20 mile trip home in the dark I realized there was smoke boiling out from the undercarriage.  

 

Long story short, the differential is now toast and has to be rebuilt.  
 

The question is - is this because of using the smaller spare tire or was it just a coincidence? 

 

 

The spare tire on trucks with 18" and 20" wheels on the 2015-2019 trucks is, from the factory, an 18 inch wheel with LT265/70R18 tire on it.  Its the same tire that is on 18" wheels from the factory, and is of the same diameter or within 1/10 of an inch of the stock 20" tire, LT265/60R20.

 

If you've got a 17" spare, someone swapped it out at some point.  The 17" spare is only on trucks with 17" wheels.  You need the 18" spare wheel and tire size LT265/70R18.

[cfell]

You “can” if it meets tow rating… but the computer will not be happy.. don’t drive over 45 until you repair properly