Geez... sorry. Thought I had that in there. I'll change the post. It is around 7-8k
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I see it now.
So you had a 7 - 8K gooseneck hooked, running anywhere between 55 - 75, and the avg mileage was from the truck's computer at 7.2 mpg; right?
The truck's computer is notorious for not being accurate. So hand calculation is best. I bet you did better than the computer results.
7 - 8K is not near the max you can tow with your truck but it's a known fact that gassers don't do as well as a diesel can in the MPG department when towing. But I think the computer results are low.
Lastly, if you can keep it at 55 - 60 your mileage will definitely improve.
I tow a 5'vr with a loaded weight right at 9500 - 9750 pounds with my 6.4L and 3.73 gears. I let the transmission, in tow/haul, manage the gears most of the time. It's not a flat tow, either. It seems it don't matter which direction I go, I've always got some mountain range in the way.
Anyway...I'm normally at 8.4 - 8.7 towing and I generally set the cruise at 60 mph on the freeway. If I could tow 500 miles on totally flat or low elevation change rolling country I'm pretty sure I'd do better.