Here's my trick. Measure the exact diameter of your fully inflated tire. I make sure the tire is perpendicular to a level flat surface. Then put a level on the top of the tire. When the level is level measure to the ground for the true diameter. All you have to do is multiply by ⚻. If you measured in inches instead of mm then multiply that number by 25.4. My 26"x2" tires true diameter is 26.25". Therefore the final number I enter into my Schwinn bicycle computer is 2095. (26.25 x ⚻ x 25.4)