Mike,
From another post, here's my view on trailer width (if you didn't see it already)
The Boxcar is exactly 78" wide. Why? So I didn't have to add the running lights across the top front and back.
http://www.nhtsa.gov/cars/rules/standar ... rpstr.html
Seriously though, I picked 78" because it matched the width of the truck. The trailer axle width matches the truck as well so the trailer tires follow the same track as the truck. I thought by keeping the trailer the same width and height as the truck, it would have great aerodynamics.
BUT, this is what I found out.
I thought this much of the trailer would be in the wind, a little sliver of the front of the trailer.
What I discovered is that the air curves around the back of the truck so that about a foot of each side of the trailer is squarely in the wind. How do I know? By all the dead bugs splattered on the front and reduced mileage. You can just see the bug guts if you look carefully.
My conclusion is that narrow is better if aerodynamics is what you're shooting for. For my truck, 60" would be a much more aerodynamic profile but YMMV.
Bruce