I did a similar trip last week with my road bike mounted on the roof with Thule Aeroblades - From PA to NC, through DE, MD, VA, and NC.
Computer said I should've gotten 35-36 MPG, only got 31 on the way down. Part of that I might attribute to a gas pump in NC that wasn't calibrated properly, but I'll never know.
I did get 33.6 on the way home, though.
Joined the dark side.