I don't much about physics but I tour with a heavier guy who carries a $hitload of stuff. He carries 4 panniers, HB bag, & a rack pack for just an overnighter. On some mutliweek trips, he has been known to take that plus a trailer with gear on it.
Anyway, on decent downhills, he can out coast me fairly easily regardless how tight I tuck.
The time I almost made 50mph, I was young, very strong, skinny, had only 2 rear panniers (about the same as front panniers now so somewhat narrow), with my foam mattress on the rack and a HB. It was a massive downhill and I had a pretty strong tailwind.
That said, my "usual" top downhill speeds over the past 5 years or so are between 35 to 40mph, rarely over 40 but it does happen about once per week on average. I am a bit reckless on downhill corners due to my really old racing days and can tuck pretty narrow and low which helps quite a bit. I typically tuck my knees together under the top tube and bend my elbows in as much as possible while keeping a flat back.
All you need is a steep downhill, smooth road, fast tires (I use Marathon Supreme usually), a tailwind, and be too stupid to have a fear of corners since it always seems the fastest downhills have lots of curves.
Tailwinds, John
P.S. My fastest ever speed was when I was young and racing. We would go out occasionally into the countryside and draft behind one of the other racers who had a van. He rigged it so the rear doors would stay open and mounted a piece of plywood as a cover or top over the rear doors so it created a 3-sided "cave" which created one heck of a "vacuum". It would suck you right along with little effort (for a young racer) until about 40mph. We would switch the chainring and freewheel (old Regina) to a 58/13 combo and spin like crazy. That is one good thing about riding on true rollers is that you get very good at doing a smooth cadence. Anyway, my top speed then was 54mph which was fairly foolish considering I was probably about 3"-6" from his rear bumper. Some of the older racers (I was a teen) could top 60mph but they were truly nuts as they looked like a gerbil on a wheel they were spinning so fast. One thing is you got super hot as there is barely any wind and you are working pretty hard. However it is fun to just cruise along at 40mph.