All tires should be inflated to the pressure recommended in the vehicles owner manual. Cars sold recently that meet U.S standards will have a notice on the driver's door frame showing what the front and rear pressures should be. This should be between the minimum and maximum pressure ratings listed on the sidewall of the tire. Improper inflation may result in poor/unsafe handling, blowouts, poor gas milage or excessive tire wear.

You will need a tire gauge. You can get a cheap pen style gauge for between $1 and $5 (USD) at a gas station or automotive shop.

To check the tire pressure, locate the valve stem, take the cap off it, and firmly push the gauge onto it, and then remove the gauge and check the reading. The gauge should only hiss briefly while pushing it onto the valve stem. If it hisses more, then you aren't pushing on it hard enough, and are not getting an accurate reading.

Spare tire

Proper inflation of the spare tire is also very important. All tires will lose some pressure over time, but since the spare tire is usually hidden from view, many people overlook it. If you don't check it regularly, your spare tire could go flat, and cause further problems when you have a tire failure.

The spare tire is often different than the regular tires. If the vehicle has a full-size spare, it's usually the same as the tires that were originally installed on the vehicle, which may differ from the tires currently installed. If it's a mini spare, the difference is obvious. So, make sure to check the pressure ratings on the sidewall of the tire to find the proper tire pressure.

If you don't already know how to access your spare tire, you should check your car's manual. There are too many different configurations to explain here.

(source: Wikibooks )