Engineering is the art of compromise.
When you set the bar high at 100 MPG, you foreclose a lot of compromises.
We know 100 MPG is possible. That little rolling coffin that VW built proves it. But VW has made no move to produce it commercially. I suspect the stack-up of compromises makes the vehicle unsalable.
Tandem layout, low-slung, a glorified diesel lawnmower engine. Only way it passes US crash-safety regs is to make it out of horrendously expensive, high maintenance, unrepairable carbon fiber.
In short: a $100,000 economy car that won’t accommodate people less flexible than Olympic gymnasts and has to be coaxed up to 70 MPH. VW has been making cars long enough to know that ain’t gonna fly.
I think drmiller gave up on tandem seating too quickly. To make 100 MPG you gotta keep the frontal area down and that means tandem seating – like a F-14 had. Look at a late model Honda Gold Wing motorcycle. Renowned for its comfortable rear seat. Honda has been playing with this for thirty-odd years and have a very popular product, On a Gold Wing the passenger sits back away from the driver and slightly higher. The seating position is nearly bolt upright. She sit with knees about a foot apart.
I would start trying to make an three-season (no snow or ice) motorcycle, build on what Allert Jacobs and Craig Vetter have done. Problem is that a totally enclosed motorcycle gives bikers – even Evel Knievel wannabes – the willies. They can all visualize the thing either falling (at a stop) or being blown over (by a crosswind) on its side and the rider & passenger are trapped.
I’d start with a tilting tadpole layout.
Welcome to Tilting Motor Works
These guys are selling conversions to standard bikes. Harley-Davidson took it a step further with the “Penster.”
Harley-Davidson shows off Penster leaning trike prototypes
On the Penster, Harley moved the rider further behind the engine and sat him even lower.
OK, take the Penster to the next level, adding a passenger seat that sits as far back as the Gold Wing. Now you can completely fair in the whole motorcycle to get a good Cd. A CdA of under 4 square feet seems easily doable.
On a faired-in, but leaning trike a little height gives the rider plenty of leverage to handle crosswinds.
For an economy vehicle you don’t need anything as powerful as a Gold Wing, Road King, or Ducati Monster, but although a <10 HP engine is probably most efficient, its 40+ MPH acceleration will be sluggish and people will hate it. I’d say a 25 HP diesel is the best compromise. Manual transmission with a reverse.
My guess is we wind up with a 10-12 foot long wheelbase, maybe 50-55 inched high and not more than 40 inches wide. With these proportions we can easily accommodate the Hucho template and get a Cd in the 0.25 range. CdA in the range of 3-4 square feet. Curb weight in the range of 900-1,000 lb.
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So they sold profitable assets and then built capacity that is now shuttered. 
