Drivers are encouraged to make sure their tyres are adequately inflated, as suboptimal tyre pressure can greatly reduce fuel economy, increase emissions, and can lead to premature failure of the tyre. Excessive pressure, on the other hand, will lead to blowouts, decrease braking performance, and cause uneven wear. Please check your owner's manual for the appropriate pressure of your tyres. In the United States air pressure for tyres (sorry: tires) are expressed in pounds per square inch, often abbreviated PSI. In countries where the metric system is used, tyre pressures are expressed in Kilopascals, abbreviated kPa.