Every car has different gearing, that’s why saying “X speed is more fuel efficient” will always have problems. One car might cruise at an RPM sweet spot at that speed but they all won’t. My old 4 speed seemed to have been specifically designed to not do the speed limit, most of the commonly posted speed limits (25, 40 and 55) were all either winding out one gear or bottoming out the gear above terrible for fuel efficiency and unpleasant to drive.
Based on that, then I guess we should have speed limits that are derived from computing the most efficient RPM for a particular vehicle? Interesting concept. Not sure it would work, though. People don’t seem interested in following any posted speed limit regardless of the gearing of the vehicle they own.