I have a Honda Odyssey (minivan, with automatic transmission). It gets 29-30 mpg on the highway with no trailer.

Every year I end up pulling a trailer loaded with boats from Key Largo to Ohio, over the mountains. With two Hobie Waves double-stacked (and a couple sets of beach wheels) the mileage goes down to about 23 mpg.

This past spring on my trip north I had THREE Waves on the trailer, kind of nested, so they were still like two high. But that extra weight brought the mileage down to about 19-20 mpg, and the transmission seemed to be really struggling, even on flat road with slight rises.

So I met up with Rick in South Carolina after he finished covering the Tybee 500, and we let the motorhome tow the boats the rest of the way to Ohio, because I didn't think the Odyssey would make it over the mountains with that load.

SO, my questions are:
(1) When towing, is it easier on your vehicle if you have manual transmission or automatic?

(2) What causes the decrease in mpg when you are towing? Is it the actual amount of weight on the trailer? I would think that once inertia kicks in at steady highway speeds on flat roads, the trailer would be irrelevant (but apparently NOT).

(3) Or is it the windage (drag) of the trailer and boats that brings down the mpg?

(4) Or does it have something to do with the size of the trailer wheels vis-a-vis the size of the vehicle wheels?