As I noted 9 posts back they work better the further back - closer to the trailer you can mount them. Most of this discussion has been about their use with a fiver but given the success I had with my TT I have to believe that with an optimum mounting location they would work just as well for a fiver. For the record I could always tell when I had the optimum attack angle on the deflector as there would be no bugs on the front of the trailer....now if the bugs weren't hitting the trailer it stands to reason neither was the wind that carries them.
I really don't have a dog in this fight as we recently traded our TT in on a nice class C and sold the truck but.... unlike many here I have been there, done that and know for a fact they can and do work. Maybe 1.5 to 2.0 additional mpg doesn't mean much to some but it translated into about $400 a year (we averaged about 6,000 miles a year towing), that we had to spend on campgrounds and cold beer. :B
You can probably get the same result by slowing down 5 mpg but since I already do that (towing at 55 to 60), this was just free money after the first year of recouping my original investment. I wish I could squeeze 12.5 out of my V-10 in the C but will have to settle for 9.5 to 10 (still traveling at 55 to 60), and the freedom that comes with the 55 gallon tank. :C