Best mileage I ever got was on BRP (several times). It's 45 (posted), no lights, plenty of twisties and elevations but nothing really drastic. And you can do plenty of 50-55 from time to time. I rarely do the math, but sure I approached (or maybe just beat) 40mpg.
Unfortunately, the BRP can give you a false sense of well being. Your mileage is the best you ever had, the interstate gauge is barely moving, so you're rolling along easy not worrying about gas. But if you don't do your homework before the ride, you don't know where the gas is located when you do need it.
You finally hit reserve and realize you've got 30-5 miles to figure it out (uh-oh). For it's whole length, only a couple gas stations are actually visible from the BRP itself; there can be one right around the corner from an exit, or it can be 30+ miles from the exit you pick (or farther if you turn the wrong way at the exit).
Even superman is not going to push a Valkyrie up and down the BRP, for very long.
If you ever decide to do the BRP for the first time, go here, print it, and take with with you. You'll be glad you did.
http://www.blueridgeparkway.org/v.php?pg=50 (these are not
all the exits, just those with gas within a reasonable distance)
It's nice to NOT see gas stations (all day).