Why are there so many gears in heavy trucks？

Now on the truck, as long as the manual transmission basically have a lot of gears, if the tractor, basically are at least 12 gears, and more than 16 gears.
Transmission design so many gears, in fact, is to make different speed ratio, and thus reduce the vehicle at high speed engine speed, thereby reducing fuel consumption.

Torque is a special kind of torque that causes an object to rotate. The torque of an engine is the torque output from the crankshaft end of the engine.
Under the condition of fixed power it is inversely related to the engine speed, the faster the speed the smaller the torque and vice versa, it reflects the load capacity of the car in a certain range.
In other words, the engine output torque is not fixed, but variable. And torque, is how much force the engine can output.

In addition to making enough power, there is actually a benefit to having more gears, which is to help us save fuel. Simply put, the engine fuel saving must be within a specific interval.
If you drive the engine speed is too high, the frequency of the injectors will increase, so the fuel consumption will naturally go up. And if you hold the engine at too low a speed.
Now the engine ECU in order to meet your demand for power when you press the gas pedal, will desperately increase the injection, thus increasing fuel consumption.