I’m running 205/60R13s on all 4 corners. My understanding is that they are very close to the stock diameter/circumference. Our car has an open rear, it doesn’t seem to me that an "exact match" would be too critical?

I have never used the spare in 30 years, and hope it stays that way for another 30 🙄
However, since we do tours and other cruising, it seems as tho I should have something available that holds air. Any use would be just to get back to home base.

I suppose that the question could be re-phrased to ask:
Does anyone have a 175/70R13 spare and does it fit under the trunk floor?