Hyundai-Kia Automotive Group is thriving in the U.S., ranking first in year-on-year sales growth last month. Even so, its market share fell for two months in a row after posting a record high of 8 percent in August.

According to data released Wednesday, Hyundai and Kia together sold 53,495 units in October, up 47 percent on-year, but down over 45 percent compared to August when sales skyrocketed on the back of a U.S. economic stimulus program which offered incentives for people trading in old cars for new ones.

The group's market share fell from 8 percent in August, to 7.1 percent in September, and again to 6.4 percent in October. The continued fall suggests that U.S. and Japanese rivals have performed comparatively better than Hyundai-Kia since September.

Subaru came in second in year-on-year sales growth with an increase of 41 percent, selling 18,169 units. No other car manufacturer posted a double-digit growth rate.

Hyundai-Kia's total sales in the U.S. for the first 10 months of the year increased 5 percent from a year earlier, second only to Subaru with 13 percent. Other major U.S. and Japanese carmakers' sales shrank by 20-30 percent.