[aerohead]

Originally Posted by Big Van

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

What NASA did is what I did to my VW van in college.Rear skirts,full bellypan,and 18-inch long boat tail.She could hit 35 mpg on a good day.You can make the bottom more horizontal than NASA did.If you can deal with the extra length,they're money in the bank!