I use gas buddy also, diesel can vary in my area by as much as 30 cents a gallon, sometimes more.
I do about 75-80% highway, and usually set the cruise at 55 when driving. Over the course of a full tank (used in less than a week) I usually see 18.5 on the lie-o-meter. This includes cold weather warm ups, and a couple minutes to cool down after I get to where I'm going.
I have reset the lie-o-meter and went on a 80 mile trip, and it said 21.8. When I'm doing 55, my rpms are usually around 1600-1700.
Stock accept for mods in my sig.