Why is AT less efficient?

[MetroMPG]

You're right on all points, and "yes" to your question.

Automatics restrict how much freedom the driver has in choosing how much to load up the engine. (EG you can't short-shift an automatic.)

Also most automatics are ill-suited to being used for one of the most effective ecodriving techniques: engine-off coasting. (Most autos rely on an input shaft-driven pump for lubrication. With the engine is off, these can be damaged by excessive coasting.)

[diamondlarry]

Originally Posted by MetroMPG

Automatics restrict how much freedom the driver has in choosing how much to load up the engine. (EG you can't short-shift an automatic.)

I am by no means promoting automatics but, I think there are things you can do that will force the transmission to shift where/when you want it to.