Air Down While You Drive. TRAILHEAD Automatic Tire Deflators will automatically reduce the air pressure in a tire to a predetermined level and stop, preventing any further loss of air from the tire. They will function properly whether your vehicle is stationary or traveling at speeds of 20 mph or less. The shut off pressure on your tires may vary due to several factors, especially the flow rate of your valve stems. 15-40 PSI Range- Life time Warranty thru Trailhead. Proudly made in the USA. Includes 4 tire deflators, one tire gauge, instructions and warranty card, Zippered case.