If your tread is badly worn, or if your tires have been damaged, replace them. Don't let anyone tell you that underinflation or overinflation is all right. Bad handling Bad fuel economy. When to Check Check your tires once a month or more. Don't forget your spare tire.
Many modern vehicles have compact spare tires.
How to Check Use a good quality pocket-type gauge to check tire pressure. You can't tell if your tires are properly inflated simply by looking at them. Radial tires may look properly inflated even when they're underinflated. Be sure to put the valve caps back on the valve stems. They help prevent leaks by keeping out dirt and moisture.