This morning I decided to compare the circumference based on calculation versus actual. For the workout app I'm writing I used the standard equation 2 x pi x r. For pi, I'm using 3.1416. 2 x radius is the diameter, so I add 622 + (2 x tire height).
In the case of my giant running gatorskin hardshell 25mm, that comes out to 622 + 50 = 172mm. When I plug the numbers into the equation, I get 2111mm for the circumference. When I measure the actual circumference using the rolling technique, I get 2140mm. That means a delta of 29mm. In percent terms, the calculated circumference is off by 1.4%.
