The United States is one of only 3 countries to not officially use the International System of Units (others include Burma and Liberia).
The Metric System is the international standard. Car manufacturers already realized that having similar parts in different measurements for different countries was a waste of resources, so all cars are now built using the Metric System for redundancy eliminations and cost reductions. Furthermore, all goods exported outside of the United States have to be label in metrics, or else they can not be sold. NASA lost a $125 million dollar spacecraft, called the Mars Climate Orbiter, over the planet Mars, because one team was using the Metric System and another team was using the Imperial System.