In 2006, a Swiss engineer built an enclosed, streamlined, 2+2 wheeled motorcycle with Cd=0.19 and CdA=0.10m^2 (1 ft^2). That's 1/20th the drag area of the Honda Insight, and the best I've ever seen for a two-seater.
Unfortunately, it was not initially concieved as a green or remotely affordable vehicle. It was fitted with a 350HP motorcycle engine for enormous acceleration and the hope of 300mph trans-continental travel. It was supposed to cost half a million Euro. As far as I can tell, that variant never got past the prototype stage.
More recently, its designer has proposed an electric version with a better coefficient of drag and a 300mi range on 15KWh of batteries. The proposed price is still astronomical.
I'd like to see a $20k version, made of steel and fiberglass instead of aluminium and carbon fiber, and with a modest small engine instead of a tuned superbike engine. Such a vehicle should be capable of at least 170mpg at 65mph with 3.5HP, or 70mpg at 120mph with 15HP.
I doubt the Acabion itself will ever be available, but it goes to show you: you can package two people and a powertrain in a streamliner with a CdA of 0.10m^2. That is, if you're willing to give up crash safety, and stability on snow and ice.