Due to variations in manufacturing, the as-built 'pneumatic drive tire" will measure different than OEM specifications. Moreover, any loading applied to the tire may cause significant radial distortions at rest that may go away at rated velocity due to centrifugal force.
I recommend you use the manufacturer specifications. You may consider doing your calculations with the specified largest and smallest radius of the tire.
The circumference is equal to:
Pi x d
2 x pi x r
where pi = 3.14.157 and d = diameter and r = radius