If the air dam is for the Ranger,I'm giving you a link to the NASCAR Truck Series.
The race cars are below Cd 0.29 and the trucks only suffer from the bed vorticity and turbulence.
The noses are essentially 'ideal' although we must allow for approach clearance with driveway ramps etc..
Your air dam should move the air around and over,never under if you can avoid it.
If the air dam must be underneath,it still must get the air directed to in front of the tires.
http://www.jayski.com/schemes/2013/t...emleftside.jpg