The numbers given were, of course, just rough (gu)estimates based on common vehicle styles and driving habits - and on US ones at that. Of course you can do your own math with more appropriate dimensions.
Interstate highways here are generally at 70 mph limits, with about 10% of drivers speeding. A few places actually do have 75 or 80 mph limits.
Since about the only times I've been out on the freeways recently has been with our 4,000 lb trailer in tow, my "Interstate speed limit" usually never exceeded about 60 - due to engine heating (6 small hamsters) with the towed load.
On "back roads" where there aren't multi-lanes to allow faster traffic to just drive by me, I usually aim for at least 10 mph below the speed limit. If I'm 5 mph below the limit, it's often very difficult for anyone to pass safely - at least without exceeding the limit, but with 10 mph of "slack" the ones who must go faster usually can find a place to pass easily and safely within a short distance. By driving a little slower I'm less of an impediment to traffic than if I drove nearer the legal limits on our 2-lane roads.
I've just had to replace my '95 Chevy Astro. At fourteen years old it was just having too many problems to be feasible to keep it running, despite <98,000 miles on the odometer. The replacement has better fuel economy ratings, but lower rated towing capacity; so I'm having to do some careful calculating to figure out what kinds of loads I can carry in the tow vehicle when the trailer is following. The rated towable load (4,900 lb) is significantly lower than the 6,000 lb rated capacity of the old truck; but that rating isn't really the problem. The new truck has bunches of "gross load rating" but there are individual limits for each axle that are going to be "really close" to the axle (and tire) limits, unless I'm pretty careful about load distribution.
I'll need some pretty significant metal mods to make things work the way I hope they will; but for now it's all theoretical mathematics and SWAGging until I can assemble all the specs and do the calculations.
The new cargo hoist is all painted and prettied up, and it probably gets bolted in the back end tomorrow; and I have already added a cross-body toolbox for some "lockable" stowage; but I still have to figure out how to cover the open truck bed so her scooter doesn't get rained on, and come up with adequate "entry assists" so she can get in the cab. (And with all the new electronics in the new one, figuring out how to change the radio station might be a good thing to get done sometime soon.)