
Make sure the tires are "cold" (vehicle hasn't been driven for more than 2km) so the air in the tires isn't expanded from heat.
Look in the owners' manual or (on most vehicles) on the inside of the driver's side door for the standard cold tire inflation pressure. This number is the lowest PSI one would inflate the tires to and is suggested by the car's manufacturer. Read below for reasons inflation may be placed higher.
Unscrew the valve stem cap from the valve stem on the tire. The valve stem is a black pencil-sized extension near the hubcap, about 1" (2-3 cm) long.
Press the air pressure gauge onto the valve stem and record the reading given. If there is a hissing sound, the gauge is not tight enough for an accurate reading. The angle...