There was a site by a young mechanical engineer who built a number of aerodynamic aids for his truck. He has since taken the site down unfortunately.

He made a topper out of thin plywood and fiberglass that was tapered from the cab to the tail gate. 17 degrees was the number that sticks to my mind. He also had a rounded piece that fit on the tail gate. The air leaving the truck is far more important than the air up at the front.

He did have a flat wrap around spoiler on the front that was nearly on the ground. There were air dams under the rockers. Moon hub caps and rear fender skirts.

He also blocked off the grill opening as much as possible. At the end of the article he was working on the belly pan.

The result was he went from about 21 mpg (small truck) to well over 30 mpg.

It was pretty interesting.