Back at the time of the "first" energy crisis, in 1973 and following years, the
speed limit was lowered to 55 miles per hour. Truckers all across the U.S.
complained bitterly that this would destroy their livelihood, and that it was
based on a fallacy, that - for various arcane reasons - the trucks were more
efficient at higher speeds.
Their protests were so vociferous that the federal government finally carried
out a real test with real loaded trucks and real truck drivers, sending them
out over a route on several hundred miles of interstate in Virginia, some each
sticking pretty much to 60 mph, 50 mph, 40 mph, 30 mph, or some such scheme.
The 30 mph speed was the most efficient (surprise!), followed by 40 mph, 50
mph, etc.
That pretty much shut up the truckers, who didn't like the implications. It
could have been worse: they could have tried 20 mph, too.