In the real world I agree on the 6.5 to 10 mpg, all depending on vehicle, your foot, winds, weight, tires on ground.
But if you want really great mileage to brag about, use 8 one way and 7 the return trip for 15 mpg over all, you might be surprised how many must figure it that way from the stories I get at the fill up pump. Have mentioned before that my standard response is, "You best get it fixed, everyone else claims 19-23 mpg on your size rig" He thinks I am as stupid as he looks, we both smile.
You just gotta have some fun sometimes.
Dave M