無論是汽油機還是柴油機，發動機的曲軸都只能朝一個方向轉動，也就是說車輛在沒有變速箱的情況下只能朝一個方向行駛，變速箱可以實現車輛的前進和后退；

　　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，由于汽油和柴油發動機在一定轉速范圍內都有扭矩平臺，車輛在日常行駛中需要足夠的扭矩來加速，需要避免發動機高轉速帶來的高油耗。

　　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.

　　這兩種需求之間的平衡應該通過變速箱來實現。 變速箱的傳動比范圍是指：1. 手動變速器的傳動比等于：從動齒輪齒數高于主動齒輪齒數；2.最高檔作為直接檔的車，最小傳動比為1。 如果有超速檔，可能會小一些，大概0.7-0.8；3.最低配的車就不一樣了。

　　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.