The figure compared with April's 11.02% year-on-year drop.
In the January-May period, car production declined 16.88% from a year earlier to 644,951 units, it said.
The federation has said car production has been down mainly because of lower pickup truck output, reflecting weaker demand as banks have tightened auto lending due to high household debt.
Domestic car sales declined 23.38% in May from a year earlier, after a drop of 21.49% year-on-year in April, the federation said.
Thailand is Southeast Asia's biggest autos production centre and an export base for some of the world's top carmakers, including Toyota and Honda, with pickup trucks among the key vehicles manufactured.
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