My experience has been the same!
Loved my F150 for road trips. All the power in the world and never needed to shift down going over 8000 ft mountain passes. And with a 36 gallon gas tank I didn't have to stop to refuel constantly. 400 miles since filling up - and over 400 miles left till empty!
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But hook a 16' trailer loaded with my side by side on it, and the mileage dropped drastically. If fighting a headwind I've seen it drop below 12 mpg.
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On the other hand, if I hook that same trailer to my old Dodge/Cummins the fuel mileage drops from 19 mpg to no less than 16 mpg!