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What does: AWD + 3 old tire (31K) + 1 new tire equal?

My AWD (Highlander, 05) has 31K miles, with 4 tire, and now I have a new full spare.

Questions:

1) Can I use the new tire as a spare? (n.b.: (in a subaru) I once mixed 3 old tire + 1 new tire…and could not used 4wd, I had to buy all new tires—because of one damaged tire);

2) Can (should) I include the new tire in a rotation?

3) Is there a rule-of-thumb of this subject?

4) Should I always just buy 4-5 new tires to keep them matched?

my rule of thumb with all the AWD Toyota’s I’ve had (5 in total) is put on and rotate 4 tires and use the spare (as just that) to get you to a service station within 50 miles. others may have a differing opinion.

I’d do whatever the owner’s manual says regarding tire rotation. Some systems are more sensitive to tire matching than others. As you know, Subarus are VERY sensitive to this, but your Toyota may not be.

Read the manual.

In all wheel drive (AWD), the most important factor is the diameter of the tires. They must be very nearly exactly the same diameter; else, you face very expensive repairs, in a short time.
There is more than one way to skin a tire, you can have a shop shave the new, or less worn, tire to the near-exact same diameter as the most worn tire.

I had this issue with a Subaru Outback, when I googled transfer case and tires, or some equivalent, I learned that most manufacturers recommend all 4 tires be the same. I ended up putting my 4 snow tires back on instead of $500 plus for a new set and traded earlier than I had intended. I questioned the shop about the use of small spare tire supplied by the man., they said for under 50 miles is its purpose.