[kingaillas]

>My car for example has four tires each at a pressure of 32 PSI.

The air pressure of your car/bike tires doesn't matter - all tires could be flat. Try comparing (vehicle weight + your weight) / (vehicle contact surface area) where vehicle is either your car or your bike.

[tzs]

For tires that support the vehicle via air pressure in the tires, contact area is roughly weight supported by tire / air pressure in tire [1].

[1] http://ffden-2.phys.uaf.edu/211_fall2013.web.dir/connor_matt...

[ars]

When the tire is flat it is being supported by the sidewalls and by the wheel.

And if you notice the contact patch of a flat tire is much larger.

The ground pressure of any vehicle is almost exactly equal to the psi. With the exception of force carried by the sidewalls.