On the left is what the Bridgestone Potenza tire now mounted on the driver-front looks like, along with the new spare placed beside it.  It's the most worn tire on my Prius.  Next to it is my never-used full-size spare tire to make comparison easy.  The tires will be rotated again around 22,500 miles (consistent with the recommended 7,500 mile cycle).  I expect to be able to use this set of tires until reaching 30,000 miles.

On the right is the tire now mounted on the passenger-front.  It has the same amount of wear as the current driver-back tire.  There's still a decent amount of tread left.  But the remaining tire, on passenger-back, is in even better shape.  I'm not sure why, but having that one rotated to the front before Winter begins will be great for snow & ice traction.

This is my full-size spare tire when stored in the trunk.  It fits surprisingly well.  All I had to do was remove the jack tool clips to make the floor-cover lay flat.  So if you look closely on the right, you'll notice I secured a tool using duct-tape instead.  Also, this shows what the rim I purchased looks like.

I waited until the tires got rotated to take the remaining photos, since being able to turn them makes it much easier to get close-ups with good lighting.  So this is what this what they look like 2,428 miles later than the previous photos of the other tires.

On the right is what the tire that used to be on the driver-back looks like now.  It's the one currently in the best condition.  Tread wear isn't a concern on this tire.

On the left is what the tire that used to be on the passenger-back looks like now.  It's condition is very close to what the previous passenger-back is currently.  So basically, I only have one tire that is wearing quickly; but it looks as though it will make it to 30,000 miles just fine.

Note that I never turn the wheels when the Prius isn't moving.  Rolling tires greatly reduce friction.  Following this advice is a great way to avoid excessive wear.

In summary, my tires are aging nicely.  Other owners haven't been so lucky.  I believe the primary secret to my success is I have chosen to maintain (which I check every 3 weeks or so) higher than recommend tire-pressure:  38 PSI front  &  37 PSI back