Well, if you really want to know...
Don't know about pre-1200's, but it can be done on 1200's with EXTREME caution (and a bit of luck, and more knowledge than I have).
On my 1200 hack I wanted to fit a
car tire. I get less than 5000 miles on a rear E3. I talked to (forget his name) the tech at Motorvation sidecars. He has fit a VW
tire onto his 84 1200, no problem. I was there (his shop in Iowa) a couple weeks ago on a trip to Minny Soda and saw it myself.
About 6 months ago (after I first talked to him about this) I tried fitting one to my 1200 rear rim. Went with a Vredstein 155R15, an equal to the VW
tire size.
NOTE: With a bit of searching, you will find many warnings about putting a
car tire on a 1200. The bead area is slightly greater in diameter than a
car wheel of equal size; I read somewhere that years ago (when GL1200s were actually in vogue...) that a machinist gentleman actually had a jig built to take several thousandths off the bead, while the wheel was ON the bike and using the engine as a motive source. He did this at Goldwing rallys; or so I read...
So I bought the
tire, and took it to a local shop. I warned him that this may/should work, but we needed to be real careful, as some had to use extreme pressure in order to seat the bead. He said "no problem" and said he would be real careful. I was watching/helping him mount it, and he got interrupted several times. So when he went off talking on the phone I went next door to a machinist shop as I had business there. Shortly thereafter, we heard a POP/BLAM, and the building slightly shook. Then a whole bunch of cursing.
Went next door, and the
tire guy was holding his very red/swollen hand up in agony, bouncing around and cursing up a storm. The
tire looked like this:
We got him loaded up on advil to combat the swelling, and cleaned the cut. He was a big muscular guy, and was very very lucky. Nothing broken, but badly bruiosed and swollen.
So. Yes, it can and has been done; but I will never try it again on a 1200 stock rear rim.
However...
A gl1500 rear rim and pumpkin will fit on the 1200.
It is 16", and several 1500s have used a Vredstein 16"
car tire on that rim with no problems. While the 1200 swingarm will fit the 1500 wheel and a bike
tire (I am using an Avon Venom), unfortunately, the 1200 swingarm is NOT wide enough to accommodate the squared off
car tire on the 1500 rim. I've tried. What needs to happen is that a 1500 swingarm needs to be adapted to the 1200 frame. It will fit, and I have my mockup done. But you need a good welder and ALSO have to relocate the port side muffler bracket, as it interferes with the swingarm.
As soon as my hack
tire wears out, this is the way I'm gonna go.
YMMV, but be really careful if you pursue the CT on the stock 1200 rim. I remember that a truck
tire (granted, our rims ara little smaller) has the potential energy to throw bowling ball half a mile. So maybe the 1200
tire could only throw it 1/4 mile...
e
and I understand it; people have preferences. 
Some like Pepsi, some like Coke. 
