Only measured fuel consumption once on my 2005 4.2 TDi ex Telstra ute with the pod on the back and got 16lt/100km. I'm still running cheesecutters. haha.
99.99% of my driving is on country roads and is mostly driven in a head/cross wind. It doesn't help that the pod protrudes externally of the cab, as it basically acts like a wind anchor.
That's probably why I sit on about 90k's everywhere I go. lol.
Rossco.