Reading the Sidewall
The following diagram shows you how:

Tire Pressure Monitoring
US (NHTSA) safety regulations require all new 4-wheeled vehicles weighing 10,000 lbs or less to have a monitoring system detecting a 25% drop in pressure on 2008 models. Tire Pressure Monitoring Systems (TPMS) measures tire pressure using a small sensor inside each wheel. When a pressure drop of 25% or more is detected a warning light on the dashboard comes on. Tire sensors are usually not wheel or position sensitive. You need to check each tire to see which one is low. For the same reason, tires with TPMS can be rotated. The sensor will need to be programmed if the tire is removed from the wheel. It is a good idea to purchase a second set of sensors for each set of wheels and tires, e.g. for snow wheels and tires, racing wheels and tires, etc. The sensors usually do not like the emergency roadside tire inflation and repair cans that many people carry in the trunk because the sealant could clogs the pressure pinhole of the monitor. If clogged the sensor will need to be replaced.

Tire Size Calculator
To help you choose tire sizes we created this Tire Size Calculator.
1. Input your stock and new tire sizes in the first and second row respectively
2. Click "Calculate" to see how the new tire compares to the stock tire.
3. Adjust the parameters of the new tire to closely match the stock tire's diameter.
(The graphics shows the stock tire in black outline, and new tire in solid blue.)