step-down gear

例句：

1. 1. The car's engine has a step-down gear that allows for easy acceleration.
   这辆车的发动机有一个降速齿轮，可以轻松加速。
2. 2. The train uses a step-down gear to reduce its speed during the transition between tracks.
   火车使用一个降速齿轮来在轨道之间转换时降低速度。
3. 3. When driving on steep inclines, it is important to have a step-down gear to prevent the engine from overheating.
   在陡峭的坡面上行驶时，重要的是要有降速齿轮以防止引擎过热。