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

BlueMax 23 · September 13, 2023

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 · September 13, 2023

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 · September 14, 2023

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.