Safely deflate a tire by depressing the relief valve located at the top of its valve stem. Tires are commonly bled or aired down to one-third of their standard highway-driving pressure to allow for better traction when driven in snow or sand. Bleeding down works by increasing each tire's footprint.
Consult the auto manufacturer's specifications located on the inside seam of the door panel to find the recommended air pressure, as the numbers embossed on a tire's sidewall are maximum inflation values. Tire pressure is measured on cold tires for accuracy. Wait three hours before testing the tires on a warm car.