Speed kills mpg because of aerodynamic drag. There are enough posts on different web sites where guys with enclosed trailers that say there is not much difference whether they run lightly loaded or heavy. They canít all be full of crap, can they? This shows that aerodynamic drag has a more significant effect than weight (with in reason) when lots of highway miles are involved. And an open trailer with a trail rig would be dirtier aerodynamically, so mpg will be reduced even more from going faster.
Flat towing should give better mpg, unless there are serious losses due to the towed rigís driveline friction or tire rolling resistance (or tweaked toe settings, /-\ lol). A flat towed vehicle is tucked in nice and tight to the tow rig, the perfect drafting. Put the towed vehicle on a trailer further back from the tow rig and higher up as well and yeah the aerodynamics will make a huge difference. Again this is assuming we all have a significant amount of highway miles to travel to get to the wheeling spots.
I not sure I understand the concern over single axle. Maybe a single axle just needs a good trailer design and is less forgiving to improper loading? I have a single axle deck over with wood sides that weighs 2k empty. Iíve towed it on many trips, with a few different trucks, from a couple miles to over 600 miles, one time full of logs, another time a pump and motor that weighed around 4,500 lbs, all sorts of crap over the years and never had an issue. All plain weight carrying (no WDH). Granted the cog of those loads were somewhat lower than a trail rig, but one could easily design a deck to bring the cog of a trail rig to the same height. What am I not getting?