MPG question

[F350newbie]

Now towing I get 8mpg at 60 and 9mpg at 65, I dont know why it is that way but it does get better mpg at 65.

May have to do with your fuel/power curve....when truck is in the "sweet spot" of the power/torque curve you supposedly get the best mileage...so I am told by diesel mechanic neighbor.