Another tip that most environmentalists will scoff at, is to paint some used engine oil on the area you want to test for use with vortex generators. Then go drive down the highway for a few miles. The wind will leave a oil pattern on the vehicle showing you the air flow pattern. (works like a wind tunnel only with used oil) So you know where to place them, and how to angle them. Plus its easy to retest to get things perfect. I learned this from a friend who races airplanes.
