The requirement of torque in running an automobile is not linear, as the static friction is more potent that dynamic friction getting a vehicle in motion needs more amount of torque than to keep it running. the engine is not capable of varying the torque delivered at the drive shaft, it can only control the power with respect to the throttle.
The above graph depicts the torque-vs-RPM for different gears. It can be easily deduced from the graph that the lower gears give higher torque but lower RPMs. this is because Power is a product of torque and speed, one can be enhanced only at the expense of the other.
The power developed by an engine is constant for a particular throttle position. So in order to get more Torque (starting torque), a lower gear is engaged. For climbing to higher speeds the torque delivery is not the primary factor, its the rotation of the drive shaft, so the higher gear is engaged.
After getting a grip of why it is done, let’s delve into how it is done. The manual gearbox has two shafts each having a set of gears engaged with the other shaft. One is called the main shaft or drive shaft other is called the layshaft or the driven shaft. As engaging and disengaging the running gears causes undue stress on the gear tooth, constant mesh gearbox is universally used now. In the constant mesh gearbox, all the gears are in mesh and are always in motion. The dog clutch is connected to the gear shifter, which selects which gear combination would be giving the drive to the main shaft.
Now would be a good time to be introduced to the concept of overdrive, the gears have constant gear ratios which drive the wheels at a particular speed. the speed of the engines are limited, 5000-6000 RPM for petrol engines and 3000 RPM for diesel engines. At the top gear, most of the passenger cars have direct drive, meaning the main shaft runs with the speed of layshaft. But in the case of top-end cars such as race cars and sports vehicles the RPM of direct connection might not be satisfactorily high, so for those special vehicles, an additional gear is provided. that additional gear is called overdrive as that increases the speed of the driven shaft more than the driving shaft.