I made it by making a line of granny squares a little bit shorter than the circumference of the tire. I wanted to have to stretch it tight, so that it wouldn't be loose and kids could still swing. The sides are made of simple, striped rows, decreasing stitches in each row so that the whole thing formed a donut when the ends were stitched together on site.
I was a HUGE relief when the thing actually FIT.
The kids mostly took turns swinging, and there was only one episode where one screamed loudly at the other to the point that momma had to threaten them with going home.