How to say "gearbox" in Japanese

• 変速装置へんそくそうち
  nounCovers the 'gearbox' and 'transmission' senses — the device that changes a machine's or vehicle's speed. A descriptive compound of 変速 'gear change' plus 装置 'device', it suits technical descriptions and is less common.
• ギアボックス
  nounA direct borrowing of 'gearbox', covering the 'gearbox; gear box' sense: the enclosed gear assembly as a physical unit. At home in mechanical and automotive contexts, though it is a less common match.
• 変速機へんそくき
  nounA compound of 変速 'gear change' and 機 'machine', it names the speed-changing unit of a vehicle or machine, with 'transmission' its leading sense; it is nonetheless a less common match.

All three are less common matches; 「ギアボックス」 is the direct loanword for the mechanical gear unit, while the Sino-Japanese 変速機 and 変速装置, built on 変速 'gear/speed change', also cover the vehicle 'transmission' sense.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 変速装置へんそくそうちgearbox
• ギアボックスgearbox
• 変速機へんそくきtransmission