As you can see a washing machine forms a nice aerodynamic shape when laid on it's side just behind the cab. It hurt my economy slightly in the city having it on there, as it adds about 120lbs. Highway mileage sees a slight increase.
I have a couple 4'x'8 sheets of white coroplast that I'm considering using over the plywood.
The typical washer is roughly 27 1/2 wide and the cab rear is about 29.
