The Honda Insight hybrid showed the largest drop in fuel economy—over 15 mpg going from 55 to 75 mphThat is just so misleading. In gallons per mile, the Insight drops from 1.93 gallons per 100 miles to 2.74 gpm. That's a difference of 0.81 gpm. The Mountaineer drops from 4.20 gpm to 5.62 gpm, a difference of 1.42 gpm.
Vehicles with lower fuel efficiency had the smallest drop. The V8-powered Mercury Mountaineer has a fuel economy of 23.8 mpg at 55 mph and that drops to 21.2 mpg at 65 and 17.8 at 75 mph..
So not only are starting off at twice the inefficiency, you end up losing almost twice as much.
This is exactly the misleading nonsense a switch from mpg to gpm would fix.