I don't know the answer to this I got a bigger trailer and my mileage went up to 10 from 9. Expected it to go down, wondering if there was brake drag on old trailer or some other drag causing it to be about the same as one 2000 lbs heavier and 6 ft longer.
They build boats with longer hulls because it reduces drag. I would assume the same thing applies to trailers.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10