...One of the tasks my truck needs to do is tow a heavy load. I used a weight-distributing hitch and towed an 11,500lb load (heavy tractor on heavy-duty equipment hauler trailer). ...With premium in the tank, the truck pulled the load up the hill well. I was at 5k RPM and the truck was able to gradually increase speed.
Fast forward to a week ago; I needed to pull the same load up the same hill. This time, surprise, surprise. With the pedal to the floor, I could not exceed 50mph. What was the difference? I was running 87, and had run a couple of tanks through.
In both cases, the truck smelled hot although all temp readouts were very good. All fans were running. I left the truck idling while I unchained and unloaded my equipment.
So, during extremely heavy towing (for this truck) I believe the higher octane is important. This is a time where the truck will indeed see sustained high RPM, and the difference was noticeable. This summer, I was pulling the same load with my 2017 Super Duty dually diesel, which was effortless. The EcoBoost does not begin to move this load with the authority of the diesel, especially at low RPM. You can really feel the engine work, despite more horsepower than the current Ford diesel and nearly as much torque as a diesel of just 11 years ago.