^^ it's not to terrable, stay with me here
. It's the circumfrance around the tire. a 28" tire has a roll out of around 88".
actually try to stay with my spelling lol. But yeah take string, tape it to center of tread and roll tire over the string till it wraps back around the tire as it rolls down the road. Measure string and you have roll out.
. There are tapes for measureing roll out (they thin, and have a hook for a tread marker, but yeah)
say one tire has roll out of 88" and other tire has 91".. That will steer a car very badly, want them to be as close to matched as possible, I always tape mine, and you can fudge a lil by airing them up and letting them stretch. but once you heat cycle them they are pretty much where they are,