I have a 24" schwinn unicycle.Its still has the original (25x1 3/4).Ok so my ? is can i use one of my old mountain bike tire on my uni? 
What are the dimensions for your MTB tires? If they fit 24", I think they will. But if you already have the tires, why don’t you try it? It doesn’t cost anything. 
mountain bike tire is (24x1.75) same as my uni tire…i think i will try it later to night after i am done rideing for the day
…Unless the tyre clearance is too small… Try it out! 
i think it should be fine if the tire stays on the rim(i was told it wont…idk why,but i will try,and post how it went.)
24x1 3/4 is not the same tire size as 24x1.75. Sure, all of the math classes you had would tell you that it’s the same size, but the first one is a Schwinn proprietary size, and the second one is an industry standard size. I’m pretty sure that the only tire you will be able to get to replace the original tire will also be an S7 tire like the one you are taking off the uni. Unfortunately you can’t just put a new rim on very easily either because the hub on the old Schwinns had 28 spoke holes
The upside is Niagara Cycle has a 24x1 3/4 S7 tire that you could pick up for under $8.
Unfortunately 24x1.75 is not the same as 24x1-3/4 in the tire world. Niagara Cycle has some inexpensive choices in this size: http://www.niagaracycle.com/advanced_search_result.php?keywords=24+x+1+3/4&x=0&y=0
I am wanting more of an off-road tire then a street tire.
There are four different tire bead sizes for which are listed as 24"; 507mm, 520mm, 540mm, and 547mm. They are basically incompatible; if you try to put a tire with a 520mm bead diameter on a 507mm rim, it’ll blow out.
So, figure out your bead diameter; it might be printed on the tire sidewall. On a Schwinn, it’s probably either 540mm or 547mm. You need to find a tire with the correct bead diameter; once you know what it is, you can do a google search for it.
That may be very, very difficult to find. If I remember right there was a Carlisle Aggressor S7 tire, but I don’t know if they made it in the 24" size. That would probably be the only tire if it’s even available. The other thing to consider is that even if you can find one that is in good 30 year old condition, it will be expensive as a collector piece.
If you want an off road tire for an old Schwinn you might look into getting a standard 24" BMX rim with a 28 hole drilling. If you rebuild the wheel you can use any standard 24" knobby tire.
Another option would be if you could find a Schwinn wheel with the blue rim from the mid '80’s. Those used a standard sized rim, and will go right into your frame.
For all of the effort it would be well worth it for you to consider getting a different uni. The Schwinn’s are good for learning the basics, and maybe for tooling around, but for more than that there are better choices.
i have looked in to geting a new(er) uni,buy money.
This is probably the best solution for buying tires for an existing old Schwinn. Second best is to find a tire for it, but then buy a more modern unicycle to replace it, and have the Schwinn as backup, and for teaching your friends how to ride.
Unfortunately that won’t work, as those blue rims were 36-hole (it was the same rim as on the Miyatas of that time). Schwinn had made all these improvements to their unicycles in 1986, but then when they came out of reorganization again, in the mid 90s, they went backwards and did away with several of the upgrades. Today’s Schwinn has modern bearing mounts and standardized hub and rim, but still has the split frame where you can only adjust the seat in 1" increments. Can’t recommend it to anybody.
It should work. I know that they are 36 hole, but I was suggesting a whole wheel which has the same bearings with the circlip retainers. Those wheels do fit a standard tire. It would be only a little easier to get one of those than it would to get a knobby S7 tire though.