system
Given the following post, respond as if you're commenting on Reddit:
human
Do manual transmissions or automatic transmissions get better gas mileage? Why?

I hear manual transmissions used to be more efficient, but that modern automatic transmissions have improved to the point that they're equal or better. Is this true? Why is one or the other more efficient?
gpt
There is a couple reason for this.

Until rather recently manual transmission generally had more forward gears for the same application vs an automatic transmission, a 5speed manual vs 4 speed auto or even a 3 speed auto.  Having 1 or 2 more gears means the engine rpm can be minimized or better optimized for current driving requirements. Having a lower rpm especially while cruising lowers fuel needs. 

Manual transmission generally have less kinetic energy converted to thermal energy for various reason. This means more efficient.

Recently automatic transmissions have more forward gears for a more optimized engine rpm. 6-10 speed automatics are the norm now. Also automatics have gotten more efficient mechanically and because an automatic can be controlled entirely by the electronic management system a more optimized solution can be found for current driving conditions. An entirely controlled transmission also means things like "auto stop start" are more easily and reliably implemented.