Routinely check your tires' pressure with an air-pressure gauge. Add or reduce air to the air-pressure value recommended in your automobile's manual. Over-inflated tires will poorly distribute your car's weight. Under-inflated tires will make your car handle badly. Both conditions can require early tire replacement.
You may have heard about a popular penny trick to check your tires' tread wear. Today's tires provide an alternative method via wear bars. Spaced evenly across a tire's grooves, these raised bars indicate the minimum amount of tread for safe operation. When a tire's wear bars show no rubber above them, you should replace the tire.
