Mathematically, circumference is PI times diameter or
3.14159..... times
the diameter.
Doesn't that depend on the defintion of 'distance' in that a circle is the
set of points in a plane equidistant from a given fixed point? Using the
'normal' definition of distance you do indeed get the above (as most of
use have known for many years...) but there are plenty of other ways
to define distance.
Not that this matters for the TU58 where the above certainly holds.
-tony