Took a 10k trailer bumper pull, plus about 1k in the bed from Southern California to Dallas Texas last year I was averaging 13mpg 65-75mph. Early '04 CC LB SRW 4x4 6.0l, 99% stock. The high speeds didn't help at all. I avg 20-22mpg at 70mph empty.
I didn't bother with the MPG stuff on the SGII.
I use my iPhone and an app called RoadTrip.
Just enter the odometer and the gallons.
There is 2 ver. One is free and the other is a few $
and lets you also track maintenance stuff.
Just finished a 3000+ mile trip hauling a 40ft goose neck from Tacoma, WA to Custer, SD and returning through northern Montana.
Total gross weight ~ 29,000lbs
Tow vehicle ~ early '04 6.0 FX4, LB, SRW, super cab, mostly stock
Average MPG was about 10.5 MPG averaging 60-65mph on flat ground. MPG would drop into the 7-8 MPG on the long slogs up some of those mountain passes on I90. Mileage worked out by dividing the miles between fuel stops by how much I put in. Since I have a bad habit of filling the tank to the top of the filler neck I know I'm getting the right amount of fuel in for the calculation.
I should of posted this sooner but here are some screen snots of Road Trip
Some of the older data is missing because I did a clean setup.
Also you can export all the data to you computer and even with a
little work you can export a trip and convert it so that all you logged stop
points can be shown on Google Maps. But that does take some work.