Just came back from Seattle to Disney/SeaWorld trip.
2 adults, 2 kids, luggage, stroller, etc.
Most of the way to destination was at 75-80. Nearing LA, did a 2-hour 90mph run, which did bring the fuel economy down a bit (the computer showed 22mpg at 90mph).

On the way home, took US101 through SF and mid-Oregon. 60-70mph there, often through towns at 30mph.

When got home, the computer showed 25.0mpg over 2850 miles, with average speed 56.9mph.