Final answer:
The trailer air supply control valve closes between 20 and 45 PSI. Proper air pressure ensures a smooth ride, better gas mileage, and less tire wear. Measure tire pressure when tires are cold as per the owner's manual.
Step-by-step explanation:
The trailer air supply control valve should automatically close when the air pressure falls to between 20 and 45 PSI. Maintaining the proper air pressure is crucial, not just for keeping a smooth and safe ride, but also for ensuring better gas mileage and reducing tire wear.
It's important to measure tire pressure when the tire is cold, as specified in the owner's manual or inside the door frame, rather than the maximum pressure indicated on the tire itself. The operation and maintenance of different types of valves, including their pressure tolerances, could be a part of an engineering or automotive curriculum.