a)
Why don`t meausure the distance between the road and the axe of the weel.
If you measure too the lenght of the coil spring you can know the load of the wheel.
I think with both measures you can know the presure.
The First one is the real radius, not?

b)
Buy an Renault Laguna or better a buggy Slesser that inflates the weel too.

Bye.