Well some real world testing in... 32km highway round trip (3 secondary highways), with some blustery weather... Secondary highways, so wasn't fighting traffic, tried to keep accel/decel rates the same, cruise to keep the drift down...
Working off the dashboard lie-o-meter, here's the numbers:
Truck - no 5th, 13.2L/100
Truck & 5th - 21L/100
Truck w/ deflector & fifth 18.7L/100
Truck w/ deflector (duct tape to fill cab to deflector gap) & fifth 18.4L/100
Ran 2 tests of each configuration (except the duct tape - it only got a single test) was +/- 0.1L/100 on restest
I'll leave the duct tape off
5L of fuel on this upcoming week's run - isn't worth having to look at duct tape.