[gambiting]

>>if the tires are completely flat then they are not supporting the car and you can't use them for measurement.

Run-flat tires are a thing though - you can drive on them even with no internal pressure, there can be a gaping hole in it and you can still drive on them. So what's the pressure the car exerts on the road when driving then?

[samatman]

A hint: the area of the tyre in contact with the road is in square meters, and the mass is in kilograms.

You'll find that, if you multiply the force of gravity by the mass, and divide by square meters, you'll have Pascals.

And there's your answer.

[ars]

That's where the "springiness of the sidewall" I mentioned comes in.

Normally (with inflated tires) it doesn't play much part. But with flat tires it can come to dominate.

[mirimir]

I've done that. They catch fire, after a while.