Here is the aerodynamic ideal (impractical on a truck):
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Blisters around each tire. Here is the MG181:
Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - Sven7's Album: Aero vehicles - Picture
These are termed
wheel spats rather than mudflaps. An example on a late model car:
http://ecomodder.com/forum/showthread.php/show-me-your-wheel-spats-13669.html#post231206
My proposal is in the middle ground. Think of it as a mudflap that is cupped or curled to stiffen it so it holds it's shape. It could be applied ahead of or behind a wheel, or both. Height maybe half the tire width?