To keep these numbers in perspective, I also specifically chose a route today that mostly shielded me from the front quarter head wind on the first leg, and exposed the car to the tailwind on the return leg. That contributed a lot.
Also, getting in that flow of traffic for about 10-12 miles helped even more on the return trip: "corridor effect"
plus tail wind
plus aero mods =
It's all about aero ... and keeping an eye on the instant MPG while driving with load.