The United States’ reign as the world’s largest auto market has come to an end. China, to which the American government owes countless billions of dollars borrowed for Washington’s massive spending projects, is now the world leader in auto sales.

According to the China Association of Automobile Manufacturers, sales of passenger cars, trucks, and buses hit 13.6 million units in 2009 — the fastest pace in a decade. Here in the United States sales slid by 21%, leaving just 10.4 million units old in the same year.

Meanwhile, China’s vehicle sales increased by 92% in December, hitting 1.4 million units for the month alone. If no more growth occurs, which is highly unlikely given the country’s growing GDP thanks to cheaper labor costs, China would sell upwards of 16 million vehicles in 2010.

All in all, not a great moral booster for the (falling) economic powerhouse that is the United States of America.