Yeah, it is correct, if you cut speed on the HWY from 70 mph to 55, as was done during the last oil crisis, you reduce consumption to 62%. Assuming gearing optimized etc... Same thing as reducing from 15 mph to 11.8. This is also true of wind force generally, double you speed and wind force goes up 4 times. At 25MPH it is estimated 80% of energy goes into cutting through the wind, on flat ground.

I would echo all the warnings abouts simple analysis of load drafting, or even rolling resistance since most trailer wheels aren't as efficient as primary wheels. Also, for peak efficiency, I would argue: zipper fairing, aero bikes, longer wheelbase, hard shell paniers, and two wheels only, with the rear having a mylar disc or a carbon spoke patern. If you want to get the load in line, you don't need a trailer to do it.