GB wrote
Robin wrote

That ruling in 2001 was in the context of what could and couldn't meet a
statutory requirement in the USA for pressure monitoring in new cars that
also alerted if more than one tyre - including all 4 tyres equally - are
25% or more below pressure. It's no surprise that indirect methods
couldn't meet that requirement.

If the car has sat nav installed and available to the car's computer, as
many do now, the speed across the ground could be compared to the wheel
rotation. That could detect all 4 wheels being equally underinflated.

Maybe. gps speed isnt that accurate in the short distances.

And if you get 4 new tyres at a time, as many do, and the tyre place
underinflates all of them by the same amount, and the tyres are a
bit different profile wise to the previous set, its going to be hard
for the system to work out that they are all underinflated. And thats
the main situation where all 4 would be equally under inflated.

But, that's 2018, not 2001.