I'm not home so I can't look, but from what I remember my factory spare for my 79 is a different size than the regular tires. Also, when I tried looking for a replacement a few years ago, I know I had a hard time.

Anyone know what size it is? Anyone know where I can get a replacement?

Hey BossMan.........It would be P195/80D15...........That be the one! 

Closest available match diameter wise is 205/75R15

Adam, eBay has them with nubs still on the tire, mounted on rim for $149.79 or best offer.......shipping $39.95. Item # 360051287386 it's a Goodyear........ corvette440hp2008-09-12 09:44:28

So...get this.... after all this talk and worrying about finding a replacement tire... turns out the tire I have just needed air!! I filled it up and it's holding. The tire looks solid... no dry rot or anything obvious. This I'm good to go on this 30 year old tire, or should I still be worried about riding on it?

I always thought that a filler bottle came with the car to only fill the spare when it was needed. If you ever put the tire on your car, I wouldn't try to drive it over 35 MPH and not for many miles.  

I bought a 79 last month that has no spare.  I have an extra set of rally wheels.  Does anyone know if a rally wheel with a full size tire (255 60R 15) will fit in the spare tire well?  Did the car come with a full size spare or donut spare?

Oops!  Never mind.  I found it came with a donut spare and will find one, and a jack and a lug wrench.

I have a 79 too.  The spare it has on it is not a full size tire, but not really what I'd call a "donut" in the way we know them today.  The one that was on my car when I got it was completely out of air, so I just assumed it was old and uninflatable, so I started a hunt for a replacement tire. I could not find one that was the same size.  From what I found they just don't make them anymore.  So I gave up my search.  One day, while I was working on my Vette, I decided to pull down the spare and while it was down I put air in it. To my surprise, it held it.  However, I still have to worry about the fact that it is really really old and probably very weak.  So if you happen to find out where an exact replacement NEW tire can be purchased, please do post your findings here!

The spare tire tubs are designed to allow fitment of a "regular" sized tire/wheel combo. How else would you get your flat tire home?
However, using a larger sized(diameter or width) tire may be a tough fit in the tub. If you get a "spare" tire to have for that purpose, just be sure it is the same size as the car was originally equipped with, or you'll need to do some "modifications" to the tub mounts.
Even on my '74, which came with G-70-15 tires, the metric replacement size of 225-70-15 is a tight fit, and I had to lower the two forward mount bolts to get a spare in.