无论是汽油机还是柴油机，发动机的曲轴都只能朝一个方向转动，也就是说车辆在没有变速箱的情况下只能朝一个方向行驶，变速箱可以实现车辆的前进和后退；

　　Whether it is a gasoline engine or a diesel engine, the crankshaft of the engine can only rotate in one direction, which means that the vehicle can only travel in one direction without a gearbox, which can achieve forward and backward movement of the vehicle;

　　其次，由于汽油和柴油发动机在一定转速范围内都有扭矩平台，车辆在日常行驶中需要足够的扭矩来加速，需要避免发动机高转速带来的高油耗。

　　Secondly, due to the torque platform of both gasoline and diesel engines within a certain speed range, vehicles need sufficient torque to accelerate during daily driving, and high fuel consumption caused by high engine speeds needs to be avoided.

　　这两种需求之间的平衡应该通过变速箱来实现。 变速箱的传动比范围是指：1. 手动变速器的传动比等于：从动齿轮齿数高于主动齿轮齿数；2.最高档作为直接档的车，最小传动比为1。 如果有超速档，可能会小一些，大概0.7-0.8；3.最低配的车就不一样了。

　　The balance between these two needs should be achieved through a gearbox. The transmission ratio range of the gearbox refers to: 1 The transmission ratio of a manual transmission is equal to: the number of teeth on the driven gear is higher than that on the driving gear; 2. For cars with the highest gear as the direct gear, the minimum transmission ratio is 1. If there is an overdrive, it may be slightly smaller, around 0.7-0.8; The lowest configuration car is different.