Necessity of a gear box in an automobile is as follows:-
The output speed of the engine is given to the propeller shaft via gear box. The engine speed is having a constant speed; the automobile runs at different condition such as slow and fast, which requires change in the amount of torque that is provided at the o/p shaft of the gear box. Change in torque is basically dependent upon the speed. The speed variation for different torque will be obtained by having different pinion and gear combinations. All these gear and pinions when assembled in a casing will give rise for a necessity of gear box in automobile. Further, it also helps to a neutral position so that, engine is kept starting without moving the wheel i.e cut off of the engine speed.