This is a well written article, part of a series on vehicle efficiency. Lots of examples of velomobiles and other vehicles that achieve a low Cd.

"This series will cover current and projected land-based vehicle energy efficiency and a high level overview of the factors that determine it, such as aerodynamic drag, weight, efficiency of motive force (e.g., engine, motor), rolling resistance, driver behavior, drivetrain losses, parasitic losses, environmental factors, Passenger Miles Traveled (PMT), etc. In this first article of the series, we will focus on energy losses associated with aerodynamic drag losses, or more succinctly, “drag”."