If anyone is interested in more in depth treatment of automotive transmissions [1] provides overview of transmission technologies (AT, DCT, CVT) and analysis of their effect on fuel efficiency. It includes detailed descriptions of power losses and their sources.

Manual transmissions have one of lowest losses but their gear ratio stepping incurs engine operating point inefficiencies.

[1] https://nap.nationalacademies.org/read/21744/chapter/7#200