Sheesh! You can get that much just by putting magnets around your fuel line to align the hydrocarbon molecules for a more efficient burn!
Seriously though, you're not going to get that much from a hardcover. It helps a little, closer to 1 mpg or a little less on the highway. You still have the low pressure areas behind the cab and tailgate. The Cybertruck solves the area behind the cab and minimizes the low pressure behind the tailgate by putting it on a slope.
Testing has shown the old trick of running your open bed with your tailgate open doesn't help - it makes it a little worse. I don't understand what Ford is thinking releasing a battery-electric pickup without even attempting to solve the pickup aero problem. Maybe Ford will offer an optional sloped canopy to improve highway range?