"Andy Burns" wrote in message
...
The Natural Philosopher wrote:

we don't disagree that the rate changes. Only by how much

As there was nothing on telly tonight, I used my OBD cable to log the four
wheel speed sensors over a straight section of road with cruise control
set to 50mph, which was measured by VCDS as 77.6kph then let some pressure
out of the right rear, turned around, rinse and repeat until the TPMS
triggered.

Starting at 39psi the right rear was on average 0.1kph slower than the
other three wheels, it triggered after letting it down to 31psi by which
time it was on average 0.2kph faster than the other three wheels.

So it triggered on a increased wheel speed of 0.3/77.6 = 0.4%

and why.

The numbers don't help with that.

They do in the sense that presumably the distance from the axle to the road
dropped
by rather more than the 0.4% that the ABS sensors recorded, so the effect
must
indeed be due to the circumference changing, not the dynamic rolling radius.