Pressure

Haven't noticed this before, but I pulled up beside a cement truck the other day and saw this on their tires:

A photo of two back wheels of an eighteen wheeler with a small tube running between them that I think is for adjusting pressure on of the tires.

I'm guessing the driver can actually control the pressure in the tires to depending on how much cement is in the mixer.

I wonder how much weight those tires can support. The concept of a ton is hard for me to grasp.