It's circular in generic terms; it's a ring in that it has depth and is more or less symmetrical.
The way I see it, a tire is the same shape as Larry Niven's Ringworld; which is to say, it resembles a ring. A shape that resembles another shape . . . is that shape. I think.
Anyway, it can't be just a circle because a circle only has two dimensions.
It's merely circular. And stuff.
