I do quite a few long road-trips. Not sure if this is your idea of “good” mileage, but I’ve been able to squeeze out 27 mpg on a Ford Expedition. You have to keep it in eco mode, use the throttle really gently, and keep your speed below 60. In the past, I’ve managed 30 mpg on the older Tahoe/Suburban with the cylinder shut off feature and almost all highway driving, but National doesn’t have any more of these since the new models came out