Thanks! But you can actually just multiply the diameter by pi=3.14159265.
My 2.2 inch Continentals have a larger outer diameter than my 2.35 Nevegals so I've come to the conclusion that those numbers aren't very accurate. Plus you'll have a different in outer diameter based on the width of the rim. A wider rim will create a larger outer diamater as well. The numbers on the tires aren't very scientific.