Hi, I’m new here but have ridden for 30+ years 
. I have a Schwinn uni from the 1970s and I would like to put knobby tires on it to try off-road riding (MUni) but the rims Schwinn used back in the '70s isn’t a standard width so it looks like “normal” tires won’t fit. Has anyone successfully used tires other than the standard issue Schwinn unicycle tire on one of these vintage unis?
“Regular” tires won’t work, you’ll never keep them on the rim. The simplest solution is to find a new 28-hole rim as a replacement. But that means a new wheel build, so you’ll need new spokes & nipples too. Better yet, buy a new unicycle that’s geard for riding offroad, because your Schwinn isn’t. Something with 36 spokes.
That said, I did notice a seller with a pair of Schwinn whitewall unicycles on eBay the other day. Can’t remember if they were 20" or 24" though. Those things are collectors items!
Careful about using the term “geared” these days, it’s no longer unambiguous! 
Roak
When I started riding (after about 35 years off!) I had a schwinn 24" that I put a 24x3 on and it fit fine. There is ample width but the schwinn isn’t the best (or even close) choice for MUni; the bearings are weak (on the old models at least) and although it most likely has cotterless cranks, that hub is terribly weak as is the single walled steel rim, with something like only 28 spokes! For around $100 you can buy a torker LX which is a decent learner uni that will suffice for light offroad MUni, but you ma want to upgrade to a 24x 2.75 knobby tire. If you can afford it, go woth the Torker DX, which is super strong with a splined hub and comes standard with a wide tire.
Does the Schwinn use the 24x1-3/4 tires? They are a different size than the 24x1.75 tires. Will the rim even accommodate anything but those weird sizes?
I think the largest tire that’ll fit the 24" LX is the 2.6" Kenda Kinetics (a 2.6" Gazzoladi won’t fit).
…
Instead of getting the DX, I wish I got a LX and saved up for a Kris Holm.
Apparently it can if you find the right tire. My experiences, back in the day, with 24 x 1.75 tires never worked.
The old Schwinns (1983 or earlier) all had fairly robust, and heavy rims, double-walled. They called them “Schwinn Tubular Rims”. But the 28 spokes was the weaker part.
the schwinn rims are a different diameter … you ahve to get the schwinn tires.
Since you have been ridind for 30+ years, you must be at least 40 or 50 years old. So I am guessing that you should have a good enough job where you can afford to buy yourself a new unicycle. Do yourself a favor and buy a new KH24 while this place still has some left: http://bikeisland.com/cgi-bin/BKTK_STOR20.cgi?Action=Details&ProdID=1203 I just bought one from them a couple of months ago and love it. I also bought a KH20 from them. A modern uni is light years ahead of that old schwinn. I have and old 20 inch schwinn myself that I keep around to loan out to friends.
No such luck; he worked for Lehman Brothers.
(It’s rather presumptuous to assume that anyone who used to ride a unicycle 30 years ago has the means to spend $400 on one now).
I gave my old Kenda of that size to a friend who has an LX 24 and it fit fine. It was the original that came from my old 24" DX, but yeah maybe it was a 24x2.6", not 2.75.