Without being indelicate, consider the size of the derrieres that will be riding the swing. Measure and mark with chalk three equidistant points in the sidewall where you will attach the eye bolts. Move the bolts to the outer edge of the side wall of the tire. Identify a suitable branch that you can hang your tire swing from. To do this, set the tire on a surface that will be at a height appropriate for drilling without straining your back. These alternative give you the option of easily taking the swing down but will cost you a little bit more money.
How to Make a Tire Swing!
How To: Make a Tire Swing