On Sunday, 24 June 2018 18:16:52 UTC+1, T i m wrote:
True ... so, it's either witchcraft, or your assumption that the
*effective* circumference (how far the vehicles travels per
revolution) must be wrong?

Because of the way the steel wire is laid up in the tyre they tend to
cross and form a parallelogram. As the tyre deflects the shape of the
tyre changes and therefore causes an effective change in circumference
(more accurately, how far the vehicle travels per wheel revolution
(down to pantographing)).

The smaller the distance the smaller the effective circumference (or
effective rolling or load radius as it's known) and to the faster the
wheel will spin for a fixed ground speed of the vehicle.

Everyone seems very fixed in their ideas about this. However, a
simple experiment should be informative.

Take a spare tyre. Wrap a tape measure around the circumference
along the centre of the tread area. Measure the circumference.
Place a plank through the tyre and stand on the ends. The tyre
will deform in a similar way to a deflated tyre on a wheel.
Re-measure the circumference. The air pressure will not have changed,
so if "pantographing" has taken place the circumference will be
significantly reduced and that change will be attributable purely
to such shape factors. If on the other hand the circumference is
not significantly altered then "pantographing" is not an important
factor.

Does that experiment seem valid?

John