Turns out the right rear tire was flat. So, I drove about a mile back to the Chevron station, bought a tire pressure gauge for $1.50, filled it up to 44 psi, and in doing so noticed something weird about that tire:
Notice anything funny this picture? No? Here's another picture of the same tire...
If you look carefully, you'll notice a small band of rough material that goes all the way around the tire. It's not a groove or a gouge, I think: it's more like it's roughly textured.
Here's a good tire by comparison:
If you look at the above picture [of a good tire], you can see that it is totally smooth - there is no rough area on the sidewall. I wiped it down a bit so that you can see it's totally smooth.
So: today's questions are...
1. What's wrong with the tire that went flat?
2. Is it safe to drive the 100 miles back to Costco Issaquah tomorrow, or should I use the spare tire?
3. If the spare tire is a donut, and I'm afraid it might be, is it safe to drive on that for 100 miles?
If it helps any, this is a 1999 VW Passat GLS V6 with ASR but not all wheel drive.
PS. Here's the view from my desk in the library:
It's pretty damned nice. Wish I'd remembered to take a picture before the sun went down...