speed-shifting mechanism

例句：

1. 1.The car's speed-shifting mechanism is very smooth and easy to use.
   这辆车的变速机制非常平滑且易于使用。
2. 2. The engineers designed a new speed-shifting mechanism that can improve the car's performance.
   工程师们设计了一种新的变速机制，可以提高汽车的性能。
3. 3. The old car's speed-shifting mechanism was not reliable, so they replaced it with a new one.
   旧车的变速机制不可靠，所以他们用一个新的替换了它。