As underinflated tires run with pressure that is below the specified setting, they wear faster on the shoulder due to the additional load being placed on the shoulders of the tires. Also, underinflated tires by only 6 psi can build up internal heat that increase rolling resistance and can cause a reduction in fuel economy of up to 5%. As tires lose air pressure, drivers will experience a significant loss of steering precision and cornering stability.
One thing to remember is that it is very difficult to spot underinflation without an air pressure gauge. Take a look at the photo below to see how an underinflated tire compares to a properly inflated tire to the naked eye:
Less common than underinflation, overinflation will cause a tire to wear faster in the middle of the tire. This is due to the "ballooning" effect that occurs when a tire has too much air. Some drivers incorrectly inflate their tires to the maximum pressure listed on the sidewall. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. The most common place to find the proper tire pressure for your vehicle is the sticker on the inside of the driver door or inside the gas filler door.
For a better understanding on how to maximize the life of your tires, read "Tire Rotation Instructions," "Alignment" and "Air Pressure: When and How to Set."