I run 30lbs. on a 230 mi trip (one way) I averaged 17.5mpg. I thought that was pretty good for a lifted aerodynamic brick that weighs 2 tons. But opinions vary.
If there's a will, usually there's a way, but then you get to that bolt that you still can't get to with two extensions, two universal sockets, and a contorsionist midget trying to help you.
1999 XJ Four door, mostly stock for now, but I am in the process of building a custom plate bumper.