Why your car accelerate itself when you put it in gear?

A car doesn't accelerate *itself* when you put it in gear. The act of putting the car in gear simply connects the engine's power to the wheels. The acceleration comes from the engine's combustion process, which creates torque that's transmitted through the transmission and driveshaft (or axles in front-wheel drive) to turn the wheels.

If your car is accelerating on its own *without* you pressing the accelerator pedal, there's a serious problem that needs immediate attention. This could be due to several things, including:

* Faulty throttle cable or electronic throttle control: The throttle might be sticking open, causing the engine to rev higher and accelerate.

* Vacuum leak: A leak in the vacuum system could affect the idle speed and cause unwanted acceleration.

* Transmission problem: An issue within the transmission itself might be causing it to slip or engage improperly.

* Faulty engine control module (ECM) or other electronic components: A malfunctioning electronic control unit could send incorrect signals to the engine or transmission.

In short: Gear selection allows the engine's power to reach the wheels, but the *actual* acceleration is caused by the engine's power, not a magical self-accelerating mechanism in the transmission or elsewhere. If your car is accelerating without your input, it's a serious mechanical or electronic issue requiring professional diagnosis and repair.