Question 2: Why do many trailer trucks use wind deflectors on the top of their cabs? How do such devices reduce fuel consumption?
When a body is moving in a fluid (like air), the medium (fluid) offers a resistance to the motion of the body. In a particular fluid, the resistance offered mostly depends upon the shape of the body.
When the top of the trailer truck is flat and normally erected, the air offers a lot of resistance to the motion of the truck. Therefore, the engine of the truck does more work against overcoming the resistance of the air. And fuel consumption is high.
In many trailer trucks the cab top is designed in such a way that it is smooth and oblique. The air slips away in such arrangements more easily and offers less resistance. The engine of the truck does less work against the resistance of the air. In this way fuel consumption is minimized and there is a significant fuel saving.