Britannica Sources used in this answer

How do a clutch and transmission help an automobile start moving?

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A diagram displays six types of simple machines with labeled illustrations: inclined plane, wedge, screw, lever, wheel and axle, and pulley, each shown with real-world examples like ramps, axes, screws, seesaws, door handles, and pulleys lifting objects. — machine Six simple machines for transforming energy into work.

To start a vehicle, the gasoline engine must be disconnected from the driving wheels to allow it to start and idle in an unloaded state. The clutch serves this purpose by acting as a device to connect or disconnect rotatable shafts, permitting the engine to run without immediately engaging the transmission.12

Once the engine is running, the transmission facilitates movement by acting as a speed-changing device between the engine and the wheels. Because an engine's torque is often insufficient to move a stationary vehicle, the transmission provides necessary amplification. By selecting a low gear ratio, the transmission multiplies the engine's torque, allowing the vehicle to overcome inertia and begin moving.1

In manual systems, the clutch is used to engage and disengage gears during this process. In automatic transmissions, a hydraulic torque converter typically replaces the manual clutch, using fluid dynamics to transmit and multiply engine torque automatically as the vehicle accelerates.1