We used livestock panels from the local farm store to build our 45' roundpen and even though I love the size to work young horses in, it is definitely too small to ride in.
The good thing about the panels, like other said already, is, that you can used them for different things as well. They come in very handy if you need to seperate a paddock or need a horse on very limited turn out. And we can always buy more panels and enlarge the roundpen if needed.
My dream is, to have a solid (wood) built 60' roundpen to ride in and the smaller one for just groundwork with the youngsters... but that's still just a dream...
I do like the idea with the tires and would love to see a picture of it!