But you said you get 14 to 15mpg. And your fuelly says 12.8mpg.
Is the 12.8 an average of highway and city?
I meant 14-15 hwy since that's often the default mark people point to when talking about their MPG.
12.8 is the average over the entire time I have owned/tracked it (Feb 2017-January 2019) for both city and highway. That also includes some of last year when I was pulling a 27ft travel trailer, getting 10mpg hwy.
If you look at this "recent" chart (Late Dec 2018-early Feb 2019), you'll see the 10mpg 100% city driving dip, a 14.5mpg 90-100% hwy jump when I went to check out the RV I ended up buying, then it drops back to 10mpg after I got back "home", when I was driving the truck still while I was getting various things fixed on the RV before I moved into it and was driving it full time.
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The following link is the chart showing the entire time I've had the truck. I've pretty much always tracked it's MPG so it's pretty accurate. You'll see the constant lower numbers last summer when I was pulling the trailer. In September it shoots back up to 14.5 and 15.5 in October, when I was no longer pulling the trailer.
http://www.fuelly.com/car/ford/f-250/1987/cubey/555168/fuelchart
Oh and I should also mention, in late summer 2017 when the truck needed tires, I put cheap all terrain, which lost about 1.5-1.8 mpg for city and at least 1 for highway. The worn out highway treads definitely got better. You might notice how the lowest mpg used to be around 11.8 city and 15-16 hwy.