Auto sales in China fell 3 percent in 2018 — their first decline in about two decades and a mild introduction to even more pain coming for automakers doing business in Asia this year, according to China auto consulting firm ZoZoGo.

The slowdown is especially painful for U.S. automakers operating in China, which dwarfs the U.S. as the world's largest car market. Automakers sold roughly 28 million automobiles in China in 2018, compared with about 17 million in the U.S. — the second-largest auto market.

A combination of trade tensions and consumer jitters have stalled sales in a country that has historically been a considerable source of growth for the industry.