Interesting subject. My take: No matter what you due to streamlining etc. still does not change the fact that a big trailer has to punch a hole in the air to move forward.

For example, a 53' trailer needs a 4000 cubit foot space to exist.

Streamlining only has a small effect on the whole picture.

What do I know--- I am not a Trucker.

see size of trailers here >>> http://yrc.com/trailer-dimensions/

You are correct that frontal area is a big variable

If you look at the equation to calculate drag force..... it considers things such as fluid density, the measure of turbulence, and lots of other things

.... but simplifying

if you boil all that down into a " preliminary number"....

then you multiply that by the area (which is your point, is a direct 1:1 affect).

and then you multiply it by the velocity squared (which basically means multiply it by the velocity then multiply it by the velocity again)

which incidentally is why speed has such a huge affect

then you take the result and multiply it by the drag coefficient.... which is the consideration of the streamlining. It's affect can be huge.

Click here to see the drag coefficients for some basic shapes

http://en.wikipedia.org/wiki/Drag_coefficienta cube is 1.05... which adds 5%

but a half streamlined body.... such as a perfect race car.... is 0.09..... which takes away 91%!!!