How does the pressure inside car tyre change once the car is lifted/ put back on the ground? Imagine a car on a jack; how does the pressure change once the car is lifted (all wheels in air - not touching the ground). I'd be interested in knowing whether the pressure will increase or decrease and by how much for an a typical car.
To be more specific I aim to answer whether the pressure inside car's tyre is influenced by the mass of the car acting upon contact area of tyre with ground OR whether it is dependent on inside area of the tyre.
 A: Well, I don't know if this is as specific as you asking, but the pressure in the tire has to balance the external forces acting on the tire.  In this situation, there are two to consider: Air pressure pushing in on the tire and the amount the earth pushes up on the tire (3rd law equivalent of the weight of the car).
When the car is up on a lift, the only pressure it feels is the air pressure pushing in.  The air pressure in the tire compensates to balance this by changing volume (Ideal gas law).  This pressure at sea level is roughly 14 psi and is dependent on the entire surface area of the tire.
When the car is dropped down to the ground, you now have the equivalent of the weight of the car distributed in each of the tires in addition to the air pressure there before (this is slightly smaller since part of the surface area is not in contact with air).  So in this case the pressure is related to both the contact area and the total surface area.
A simpler approach is to look at the tire.  When up on the lift they have  a set volume.  When dropped to the ground, they are compressed.  As the volume goes down (with fixed temperature) the pressure has to go up.
A: The pressure of the tires would decrease because when the car is lifted, the force on the tires from the mass of the car decrease, causing the area of the tyre to increase. 
  Since the formula for pressure is PRESSURE=FORCE/AREA, the bigger the area of the tyre, the less the pressure of the tyre.
