What fuel mileage do semis typically get without a load?

I don't know that it is pointless to know what different trucks gets empty.
I knew a guy in East Texas years ago that bought a pete, and hired a driver to chauffer him around, it wash is limo more or less, he did not even have a trailer.
I have myself considered turning my international into a fth wheel puller. I am thinking I could set it up well enough mileage wise to compete with diesel pickups.