Spare tire question

So I'm replacing my nittos .there 3 years old,an have maybe 10,000 miles of wear left. I ruins one last year an replaced it with a d rated pathfinder which now has 8,000 miles of wear. So would you use a old nitto or use the newer pathfinder as your spare ?? Ya I'm to cheap to put a new nitto as a spare. The pathfinder worked will in its rear wheel position. Thanks. an yes most miles on truck are off-roading.

 

I would consider the manufacturing date on the tire as well as the overall condition...cracks, dry rot, etc.