Freeway speeds aren't the best
Driving from Seattle to Pullman (WA State) is about 1/2 freeway, 1/2 2-lane. IME the best mileage is on the slower sections, around 55 mph..
Wind has a lot to do with mileage on that route - at least as much as much as +- 2 mpg.
Kind of hilly, too. Best mileage is usually attained at the lowest spot (after Snoqualmie Pass eastbound).
Best freeway mileage I've seen is Seattle-Portland on I5 - it's pretty flat.