system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are automatic cars less fuel efficient than manuals (stick shifts)?

It surprises me that automatic transmissions are typically significantly less fuel efficient for the majority of everyday drivers in the most common cars in comparison with the manual equivalents. 

After years driving automatics (and a Prius - which is CVT rather than automatic, so we shall ignore this) I have returned to manuals not least to save money and have noticed the difference already. Petrol costs around $8.22 per gallon (US) in the UK at the moment.

I had sort of assumed that automatics would change gear at a point that was good for efficiency as they are not generally seen as optimum for performance. I guess I am completely wrong. Surely it cannot just be from the additional weight?

EDIT: fantastic response all, thank you very much. Will be interesting to see if the inefficiencies and overheads can be addressed in due course.
gpt
These days, they aren't. They used to be, because the torque converter was always slipping and the transmission took a fair amount of energy to run. But modern torque converters are locked when the vehicle is at speed, and they have become much more efficient.