Smallest undersized spare tire I can get away with?

[Renegade_Medic]

Another note is that 235/85 is a stock size on full size trucks. Used tires for spares can be found reallllly cheap. I got myself a Cooper ST (I think? mud tire) for 50 bucks, still with the nipples.

Yes, my 93 F250 had this size stock. They are plentiful.

 

[Tyler James Inc]

2) Even on the front and in 2WD, it is going to make the differential "differentiate." This wouldn't be a big deal going 10 miles to the tire shop after work. But, if you had a flat 200 miles from your destination on a road trip, it might cause the diff to overheat.

The ADD sleeve in the "jack shaft" of the front diff effectively separates the passenger side from the diff, so stress on the front diff shouldn't be an issue.
I guess if you have the means to use the spare on the front- regardless of which you lose- an undersized spare should be fine.
I had a pretty strong opinion against it typed out, but I swear most of those "donut" spares are smaller than the spec'd size for the given vehicle, so I couldn't really justify my opinion.

 

[Renegade_Medic]

Yeah, just because the differential is spinning at different speeds per shaft, with an open diff, I don't see the problem. Locked or Limited Slip would have some serious problems.
The Donuts also say for temporary use only. Define Temporary...

 

[Lindenwood]

The ADD sleeve in the "jack shaft" of the front diff effectively separates the passenger side from the diff, so stress on the front diff shouldn't be an issue.
I guess if you have the means to use the spare on the front- regardless of which you lose- an undersized spare should be fine.
I had a pretty strong opinion against it typed out, but I swear most of those "donut" spares are smaller than the spec'd size for the given vehicle, so I couldn't really justify my opinion.

Hrmm. Good point! I suppose if the system is meant to allow the passenger tire to remain connected to the diff, and the front driveshaft is typically not moving in 2WD, then the diff's spider gears are actually constantly in motion!

In that case, yeah, it shouldnt be a big deal at all, with respect to open diff longevity, to put a different sized spare on the front as long as you keep it in 2WD on pavement.

I cut out my rear cross member, added a new tube that sat higher up (due to body lift), and used a 235/85r16 as a spare for years. Never had to use it, but it was near the same 31 inch spare tire size, and was barely visible.

As other mentioned, if you run a small spare for a very short time, you'll be fine, but otherwise it will cause damage to the diff.

A family friend once went to get new tires at a tire shop, and ended up 285's on 3 corners and a 265 on the 4th. Needless to say, the diff locked up, and the shop ended up paying for a whole new diff.

Eep. What vehicle was this on?