I'll add this.
I have a 14L Series 60 Detroit. I don't get the best fuel mileage IMO.. I am around 6 MPG really. But I don't slow down. Unless it's a big 5+% grade. Even at 80,000. If you are in Minnesota, then you are very familiar with the hill on I90 leaving WI and going west in Minnesota. About 5 1/2 miles long and a good 5+% grade. The last two times I went up it. I had a air leak on my intake manifold. I was able to pull that hill at 65 mph in 12th gear at 1700 RPM at 74,000 pounds.
Long enough to drive back out to Baltimore and back to Minnesota later, I had fixed that air leak and sealed up the engine tight.
I pulled that same grade at the same weight in 13th gear and 74 MPH at 1700 RPM.
All I did was install a stupid o-ring in the intake.
At the intake manifold under the v-band clamp
This is exactly how on these big power motors, something so stupid and unthought of, can cause you a major headache, power and economy loss.
It's a 2006 Series 60 14L EGR motor. Running about 630 HP and 2300 lb/ft of torque. The only hard part I have "changed" from stock is the turbo. I got rid of the VGT turbo and went to a non-wastegated Borg Warner. More out of simplicity of design for reliability and cost of repair. $630 vs a $2800 turbo.
My engine started at 515 HP and 1650 torque. I was at 610 HP and 2150 torque with the VGT turbo as well. Ran that turbo for two years before changing it to the BW turbo. I ran from 703K miles to 1.13 million miles with that power. The only reason my engine was inframed was an o-ring failure on #1 Cylinder liner. Otherwise I am positive it would have gone longer for the hard parts. I virtually had no blow by on my engine. Zero oil consumption until 18-20K miles and then it was a 1/2 gallon and an oil change. Trucks will tell you when it's time if you pay attention to them.
I still use the OEM injectors and cam. Haven't changed them. I could likely go up to 800 HP if I wanted to just on my ECM tune alone without any parts changing. It would take considerable hard parts in a cummins or cat to do similar. I have different cam and injector options on the 14L if I wanted the power. The 12.7 liter Series 60 can see that kind of power. But it's got a lot more things you'd have to do to it.
Between the 14 or 12.7 L engines, the 12.7 is the fuel mizer. The 14L is the brute force. The weak link in the 12.7L is the bull gear. That was addressed in the 14L by going to a bushing instead of a bearing. The Series 60, regardless of size, also has issues with the OEM exhaust manifold cracking. Right in the center section. Just a bad part design that I think is suffering more from a bean counter forcing a part design change. There's an aftermarket manifold that replaces it that fixes it and I haven't had an issue with it since I installed it on my 12.7 truck over a year ago. My 14L got that same manifold shortly thereafter.