Overall, I've filled my tank 53 times now, and my mileage varies from about 19.5 to 21.0. In general, I've concluded that the mpg sweet spot for me is about 55-65 mph, and I lose mileage over that. On the other hand, I live in the west and 55 is just way to slow on our 70-80 mph roads.
My experience has been that your mileage will depend on the speed. Driving across west Texas on the interstate, I was getting about 16-17 mpg, but that was at 90+ mph. The best I've ever seen was a 200 mile stretch on Oregon highways where my instrumentation read 25.0 mpg. (Oregon highways are stuck at a speed limit of 55, and they really like to enforce it.)