An automatic transmission has the ability to automatically switch gear combinations based on current engine load and RPM level. Instead of a mechanical clutch, a viscous coupling called a torque converter is used. This is a separate unit from the transmission itself. There are several gears inside the transmission, and some are engaged while others are disengaged. Depending on the combinations, many gear ratios can be created. The engaging of gear combinations is controlled by internal clutches and the governor.
Torque converter amplifies torque at low RPM's, and helps put RPM into engine's powerband
Gear is automatically selected, which is usually the best one for the conditions
Automatic transmissions are more heavy-duty and more reliable than manual transmissions.
There is no clutch that needs to be replaced at high cost
The engine cannot stall, and missed shifts do not occur.
The transmission automatically shifts before engine redline, preventing possible catastrophic engine damage
These pros have resulted in automatic transmissions dominating street drag race
s (thanks-Transitional Man
Parasitic loss is higher than with manual transmission (although this has become less of a problem with lock-up torque converters)
On some cars, an automatic transmission is an option that costs extra.
It is impossible to push start a car equipped with automatic transmission
There are more restrictions on how one can tow a car equipped with automatic transmission.
It is more difficult (although not impossible) to choose a gear different than what the transmission wants