Anything that sticks into the air stream creates turbulence and ultimately an area of low pressure behind it. The area of low pressure created by turbulence must be overcome to move and object forward. And the larger the area of low pressure, the more energy required which ultimately degrades fuel economy and range in our trucks.
Remember roof top gutters on car and trucks? They created turbulence and contributed to the area of low pressure behind them. Now gone from automotive design for the most part.
I don't know where the idea of cab mounted air dams came from, but, the inventor seemingly was not aware of Bernoulli's Principle derived from Sir Issac Newton's 2nd Law. And these 2 gents published their findings in 1738 and 1687, respectively.
So none of this is new or voodoo science.
Edited by Advmoto18, 02 June 2015 - 02:38 PM.