Question Towing mileage

[got9lbs]

I just towed from MI to New Orleans and got 8mpg-11.5mpg during the trip hand calculated per fill up. I was pulling my 20ft enclosed with about 2000lbs in it. I was not happy with this mileage, was also using mobil 1 5w-40 syn. We were going about 68-75mph for anytime we got 8-10mpg. I think the only reason we got 11.5 mpg one time was were were stuck in snow in 2wd going like 45-55 the whole time. Truck is a 06 f250 crew cab 5 speed, fx4 4x4. Running 285 bfg tires. Why did my mileage suck.

 

[lomac9]

I am running 37 in tires on 22 in rims and dont have any mods done to the engine (chips,scanners ect.) I know my speedo is off which im sure makes the computer miliage off. In the process of trying to pick the best chip option for me but anyway would anyone know a approx ratio i could use to calculate my tire/rim size to my existing mpg to give me a more accurate reading? Now my miliage is saying 9.8-10.5