On Mon, 25 Jun 2018 18:20:24 +0100, "NY" wrote:

snip

But if you were to label part of the tread with a line continuing radially
up the sidewall to the centre, that line would not remain a straight line;
it would curve one way as the labelled part approaches the road surface and
then the other way as it leaves it; this happens because the plies of the
steel belt move towards or away from each other as the tyre rotates. Not by
a noticeable amount; probably by an amount that is hard even to measure.

+1

The
tyre is no longer a perfect circle but a circle with a flat on the side
which is contact with the road at this precise instant.

Yup.

So, this could be easy to measure.

With a correctly inflated car tyre on the car and parked on a
smoothish / flat surface ... jack a wheel up and measure the
circumference with a tape measure. Lower the car back onto it's wheel,
keeping the tape in place and once back on the ground, check the tape
again.

Lower the tyre pressure a reasonable percent and measure again.

Cheers, T i m