system
Given the following post, respond as if you're commenting on Reddit:
human
How does a stick shift save gas?

People always tell me that stick shift cars use less gas than automatic transmission. Is this even true? Do you have to drive a certain way?
gpt
So much mis-information in this thread.

Two things at play here - Inherent losses in the transmission, and driving style.

Last first - you can drive a very efficient manual very aggressively, and get worse fuel economy than a very inefficient automatic.

For the transmission, let's time-travel back to the 90's - most automatics were 3 or 4 speeds, and most manual transmissions were 4 or 5 speeds (5 being more common).

The automatics of that era had two disadvantages:
1) less ratios (speeds).  With fewer speeds, the engine would have to rev higher at each shift, and higher at highways speeds.  This leads to more fuel used due to higher frictional losses in the engine (engine has to rotate more times for any given distance traveled, and each rev involves the same amount of friction)
2) The torque converter.  Think of a torque converter as two fans facing each other, with one connected to the engine and the other connected to the wheels.  If there was only air between the fans, then the car would barely move, but these fans have fluid in them instead of air, and the fan blades (fins) are very well designed.  The lack of direct connection is what allows the engine to stay running when the car is not moving (like at a light), but churning through that fluid saps energy (one fan moving, one stationary).  Also, when you go to accelerate, the fans are moving at different speeds, again consuming energy in the transfer through the fluid.  Only when the two fans lock together does it become more efficient (lock up torque converter / torque converter clutch).

A manual transmission of the era would have, on average, on more gear ratio, and almost always be geared to rev lower than the same car with an automatic, especially at highways speeds, so lower revs = less friction = less gas used.

Also, the clutch in a manual has no frictional losses except when engaging, which takes very little time.  At all other times, there is a direct non-frictional lossy connection.

Fast forward to today however, and many automatic transmission vehicles have more ratios than their manual counterparts, also, quite a few have done away with torque converters in favour of automated clutches (Volkswagen's DSG is an excellent example, as is the Eaton ultrashift in heavier duty tractor trailer applications).

As many have posted, the advantage is not really there any more, depending on the car.  If you buy a base econobox with a 4-speed, torque converter equipped auto, you will still be able to best its fuel economy with a manual version.   Buy something with a dual-clutch 7-speed auto, and you will have a hard time beating it, given the same driving style.

Heavy-footed driving is not exclusive to auto or manual...

Source -fleet manager for a national trucking company.