Basic (and all): A retest may be in the works. I had forgotten about the tow of my dad's computer desk with his truck until I came back in this thread. But here's why I'm considering a retest.
I towed the same atv on the same trailer to the same place and back, same route and same speed (65 mph) with the truck as with the Mirage.
However, I found a way to get the trailer gate to lay down (forward). For the first half of the trip I left the atv all the way forward on the trailer. On the way back, I moved the atv toward the center of the trailer, in the neutral position (I think I described using a bathroom weight scale under the landing gear on this thread to find the neutral position). Let me just say, on the way back was the most pleasant tow I have done in a long time. The way there wasn't bad, but I did feel the weight on the truck. This Sierra has the Z85 (high payload, firm ride package). Which is fine for towing, but makes it ride like crap (worse than my 3/4-ton Ram) unloaded.
The whole tow there and back (~250 miles) the truck averaged 17.5 mpg. That's a 4.8L 2WD Short Bed Standard Cab with a 3.73 rear gear. I thought that was pretty darn good.
But ... since I've found a way to get the trailer tailgate laid down, AND can move the atv rearward to the neutral position, now I'm kind of wanting to do this test again with the Mirage ... with the trailer tailgate down. Maybe I could get 30 mpg...