I bought a new f150, eco 3.5,2011, 6.5 bed, 3.73 rear. I have 33k on it. Not used much. Never got good mpg as in 18-20. My average over 33k is 13.8. Just this weekend, I get good mileage, I didn't change anything. Back roads and some highway, total 66 miles, 19.4 mpg. A nice surprise. No way to explain it. I may drive it some more now.

 

If you live in a northern climate they probably changed from winter to summer blend fuel.
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Wind resistance kills fuel mileage on trucks. Driving on back roads with little stop and go traffic at speeds under 60mph will give you the best fuel mileage. Get up to 65 or 70 and you'll lose 2-3 mpg. Steady speeds over 70 will kill you as does true city driving.

And only 66 miles doesn't mean much. I once got 36 mpg over a stretch of road for 40 miles. It was all downhill, in a rural area. with no stops or other traffic. Never touched the skinny pedal and rarely the brakes. I controlled speed by shifting up and down as needed.

 

That wind resistance doesn't just affect trucks. It's the reason so many states have a 50 or 55 mph limit on state roads. Get much above that and the drag really starts to matter. The math for the force due to air drag is based on the square of the speed.

Other than the occasional observation at the pump, I also didn't really watch my driving versus fuel consumption until I got an electric car for my commuter. If you have a friend you can borrow one from, its worth the experience just to play with it. With so much less range, just a few miles on the interstate really shows how much you pay for high speeds.

 

It's not the weight of the trailer it's the aerodynamic drag that it creates. Your post gives me hope. I'm pulling a Uhaul 5x8 across the country next week. I was planning for 10-11mpg.

 

I played around with the calculator on this website: https://www.vcalc.com/wiki/vCalc/Force+of+Drag

If you assume just 15 square feet of trailer presented to the wind (the trailer is certainly more) and a speed of 75mph you get a force of 211LB. That's quite an additional constant force on the truck. It does not factor in weight of the trailer, drag from the wheels/tires, etc.

 

Sorry folks just realized this landed in the wrong thread.