On 24/06/18 22:18, Jeff wrote:
So when
a wheel with tyre sees a reduction in the distance between the
axle and the road due to a lower pressure in the tyre, you get
the same change in the rotation rate of the wheel which is
easy to measure with the ABS sensor on that wheel.

No, you don't.

a tyre and wheel is not a sun and planet reduction gear, the tyre is not
rotating relative to the wheel, ergo every revolution of the wheel one
circumference of tyre must move along the road.

The ABS sensors do not detect a massive change in radius, but a small
change in circumference due to the tread shrinking very slightly.