I'm not saying efficiency is all about the wind drag, but, it is all about the wind drag :
http://www.theverge.com/tldr/2015/9/18/9351763/worlds-fastest-bike-human-powered85mph
without electric assist!
I have a recumbent bike (Vision R40) and it is far, far faster than a road bike on everything but steep inclines, simply due to the improved aerodynamics.
It has over seat steering which allows easy use of a rear-view mirror mounted on the handle-bar. Works great. Visibility is fine, both for operator and automobiles.
On downhill sections in town, I take the car lane because I am doing 35 mph anyway.
Now it is winter in MN and I have switched to full-suspension upright. To go with a recumbent in winter, I feel, would require a trike design - otherwise your are in danger of the bike coming out from under you while your legs are up in the air.
Also, with the inattentive drivers these days, I stay off the road when possible. Sidewalks are not as smooth as the road so, suspension is very desirable.
In cool weather, a cover would be nice, both for aerodynamics and to moderate body temperature.
Now, three wheels, suspension, covering, the weight is adding up, so, time to take advantage of modern battery and brushless motor technology and turn it into an e-bike.
Here is what I would throw together:
http://www.icetrikes.co/community/ice-news/ice-full-fat-26fs... Electrify that and then use this cover that BMW developed called GINA:
https://en.wikipedia.org/wiki/BMW_GINA#Fabric_bodyMount the cover on aluminum tent pole-type material. That would be extremely light-weight and aerodynamic and leave open some options for configuration depending on weather, etc.
You're welcome ;-)