[aerohead]

I've played around with NASA's 1977 aero-modded Ford Econoline.
Adding :
*upper edge radii
*roof camber
*side body camber
*four-wheel wheel skirts
*MOON-esque wheel covers
*Wheel fairings
the drag falls from Cd 0.238,to Cd 0.13 and change.
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With the vintage 300 CID inline-6-cylinder,M-4 with OD,steel radials,highway mileage would go from 17.6 mpg (I had an Econoline and this is what I got) @ 55-mph,to 24.0 mpg.
With a modern TDI powerplant,more than 31.2 mpg.
NASA's wheelbase could be stretched,and much weight added without significantly impacting HWY mpg.
The BMW diesel-powered Vixen motorhome could do up to 28-mpg if I remember correctly.