On Sat, 23 Jun 2018 21:47:33 +0100, Tim Streater
wrote:

snip

Dunno what "rolling circumference" means, but the circumference can't
change, as it is fixed by the loops of steel belts in the tire (may
stretch a bit but not much).

But the 'belt's' are parallel loops around the circumference of the
tyre but woven diagonally and so form a parallelogram. This
parallelogram can change it's shape slightly and therefore allow the
tyre to become bigger or smaller due to pressure or whilst carrying a
load.

*That* is the point.

You wouldn't get the point unless it was stuck in you by someone else.
;-)

Cheers, T i m