free speed

例句：

1. 1.The car accelerated to its free speed.
   汽车加速到了它的自由速度。
2. 2.The plane reached its free speed before landing.
   飞机在降落前达到了自由速度。
3. 3. The train was traveling at its free speed when it hit the signal.
   火车在遇到信号时以自由速度行驶。