Tyre Pressure Monitoring Systems

On some cars Tyre Pressure Monitoring Systems do not actually measure the air pressure.
They detect significant loss of air pressure,typically 30%, in the tyre using the ABS sensor.
The ABS sensor measures the speed at which the wheel rotates and when the tyre is under inflated the wheel diameter is reduced, therefore, the wheel spins faster.

I attended two cars with such a system recently and the tyre pressures were 9psi below the recommended pressure. Both owners thought that they didn't need to check the tyre pressures as they were under the impression that the system would warn them of a small drop in tyre pressure.

The tyres on both cars displayed accelerated wear on the shoulders and the tyres will need to be replaced much earlier.

Run flats are not the cheapest tyres, therefore, if you have such a system my recommendation is to check your tyre pressures.