Whatever the semi in front is doing
Seriously, I've never found one particular speed that's most economical. It depends on things like slight up or down grades, head or tail winds, the road surface. Heck, maybe whether the spirits in that particular area decide to play tricks
There's one particular stretch of road, about 15 miles and absolutely flat, on which I can't get more than 65 mpg whatever I do, even though on similar roads, driving the same way, I'd be above 75.