Speaking for the country of Japan as a whole, not just manga/anime:
Short answer: Currently the legal standard follows SI units, which for torque is N-m.
Longer answer: Keeping in mind though that manga/anime such as Countach, Wangan Midnight and Initial D were written in the late '90s to the '00s, and far as I can tell, car culture in Japan up to the late '00s used kg-m primarily over N-m. (
watching researching old Best MOTORing clips to confirm this)
1990 (kg-m only):
View attachment 26421
2008 (kg-m with N-m secondary):
View attachment 26424
2022 (N-m only):
View attachment 26427
For that reason, I believe that most car manufacturers selling cars in Japan still put kg-m on their spec sheets, for recognizability. For example, this is from the spec sheet of a car sold in Japan in 2025:
View attachment 26415
So, the answer is basically both. N-m is the legal standard, and the convention in car culture was kg-m, but is gradually transitioning over to N-m.