I noticed my old spare on the rear door is really starting to get some dry rot cracks and probably needs to be replaced. I've actually never had a flat with my LR and the rubber that is on the spare came with my used vehicle, a Michelin MS I think.

I'm not at the point of needing new tires but will probably be looking in the next 15,000 miles or so depending on my driving conditions.

I typically only do a back to front 4 tire rotation so the question is whether to get a decent tire in the event I need it or perhaps even the same as what I will have on the road tires or just find something that will save me if I get a flat (which with the old Michelin is doubtful). Most likely the tire will sit on the door, albeit I do keep a tire cover over it so it is not as exposed to UV light.

I've considered if I wait until I am closer to needing new road tires I could just move the one with the best tread to the spare but I may change sizes slightly and by then the current tires will be about 6 years old and may be cracking as well.

I guess if I end up spending $700+ for new tires what is another $170 or so

 

Based on the age of your spare, you would do best to leave it as a spare, it will still work in case of a blow out but not for long term mileage. When you buy a new set of tires, take an old one from the ground and mount it on your spare carrier.
Buying or running a new spare is really a waste of money.

 

You can probably find a good one second hand. I've got a new 255/65R16 in my shed because when I put new tires on I put a much larger size so I had to buy a spare too, and my existing spare was unused so I kept it.

 

Craigslist is full of used tire dealers.