the measurement for tires are commonly psi, which means pounds per square inch. It is a measurement of how much pounds can one square inch of surface can support, so usually you will notice that the heavier the vehicle, the higher the tire pressure, since that one square inch of surface needs to support xxx weight of the vehicle.
You have to understand that this is doesn't exactly relate to the volume of air in the tire, simply put, bigger tires, more volume to get the 40psi mark, and smaller tires will need less air volume to reach the 40 psi mark, so regardless of what tire sizes you choose, you can stick to the manufacturers recommendations (usually located at driver door, or in the glove box) because they have already researched how much psi you need to support you car's weight without the tire sidewalls deforming to much (too much bulge).
The only reason for the pressure recommended is to prevent extreme deformation of the tire sidewall. Some tires specially designed for rally or circuit have strong thick sidewalls to avoid deformation, so you notice that the racing teams usually play with tire pressure to reduce or increase grip.
Hope that helps.
