4x4ord wrote:
When that engine is working at 1800 rpm and just up to the dark blue sweet spot as you call it it is putting out a minimum of 150 horsepower. (440 lb ft of torque) According to the graph it would be burning 7 gallons of diesel per hour. Assuming you’re towing at 65 mph you’d be getting 9.3 mpg. So in order to be in the dark blue you’re working that truck . The dark blue represents 6.2 to 9.8 mpg at 1800 rpm and 65 mph. I would expect when you’re down at that kind of mileage the automatic is going to be selecting 1800 rpm over 1360 rpm.
Edit: I didn’t do the math on whether or not you can get a tire size and final drive ratio to do 65 mph at 1800 rpm. If you’re only doing 60 mph at 1800 your fuel economy is going to be about 10% worse to get into the dark blue of the graph.
The 1,800 rpm is not the thermal efficiency sweet spot because I call it, it is the sweet spot due to the burn rate of diesel fuel. It is not like it is an opinion that I can change at will. Doing some more research on it, there have been many studies proving this. Of course thermal effiecny of diesel fuel is a little different than the point at where a certain diesel is most efficient due to rev limits and so on. However, most diesels that can rev that high, then 1,800 is generally their sweet spot because of this.
Take this slide from a study Caterpillar did for the DOE showing that their C15 engine having much higher thermal efficiency at 1,800 over 1,200 rpm. Peak torque of the C15 is at 1,200 rpm for most if not all applications yet it has a higher thermal efficiency at 1,800 rpm.
Just to tow a combined weight of 21,000 lbs(sometimes 22k) at 65 mph up a slight 1% grade requires 150 hp. Of course this does not account for a lot of wind drag which gets worse the closer I get to the coast. The automatic will try to hold 6th for as long at it can and will for most slight hills, but the boost will be high, fuel pump will be close to max, and my EGTs will be 1-200 F higher than what it would if it were in 5th. My brothers 2012 F350 PSD 3.55 did the same towing his 5ver and he locked out 5th as well.
My whole family camps out at the same RV park at the coast and we live within 5 miles of each other so we generally follow each other down when we are towing the RV's.
