Thanks, Aerohead, your explanation was really helpful. So I guess the best thing to do is to try to go from "rough brick" to "smooth brick" with a belly pan, wheel skirts, etc. I might also try something like this for long hauls where parallel parking isn't an issue:
Green Car Congress: New Boat Tail Design Could Improve Class 8 Truck Fuel Economy by 10%
I don't think I'm ready to invest in the ultimate solution:
AeroTruck E-38096: Air flow testing on aerodynamic truck