I was curious about this claim, so I did a bit of Googling.
This is difficult to substantiate, because the estimates vary so widely:
http://www.smithsonianmag.com/travel/fire-in-the-hole-77895126/
Estimates vary, but some scientists believe that anywhere from 20 million to 200 million tons burn there each year, producing as much carbon dioxide as about 1 percent of the total carbon dioxide from fossil fuels burned on earth.
With an order of magnitude between the high and low estimates, it's hard to pin it down. But, let's go with the high estimate, which says it produces about 1% of the total CO2.
Per the EPA, cars and light trucks in the US account for 56% of US transportation emissions of CO2 equivalent: https://climate.dot.gov/about/transportations-role/overview.html
US transportation accounts for 26% of CO2 equivalent US emissions: https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions
The US accounts for 14.95% of CO2 emissions: https://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions
One caution: the EPA data is CO2 equivalent -- meaning that other greenhouse gases were converted to the relative effect of CO2. But, the data from the EDGAR database (cited in the Wikipedia article) is CO2 only.
But, if we do the math with what we have: 56% of 26% is 14.56%. 14.56% of 14.95% is 2.177%.
So, the estimate of Chinese coal fire emissions could double above the largest estimate, and still be less than US cars and light trucks.
Sorry, but your claim (or whoever made it) doesn't hold up under scrutiny, unless it uses different data. The estimate I cited above is from 2005, but I wasn't able to find a more recent estimate.
That doesn't mean that it's not a problem -- just that you should be using a different metric for comparison. For instance, if you were to use only cars or light trucks (they generate a comparable amount of CO2 in the US), either alone would be comparable to the high estimate for China's coal seam fires.
I found this even more interesting that the calculation of the CO2 statistic:
"Much of the landscape of the American West— its mesas and escarpments—is the result of vast, ancient coal fires. Those conflagrations formed “clinker”—a hard mass of fused stony matter. Surfaces formed in this way resist erosion far better than adjacent unfired ones, leaving clinker outcrops."