Why diesels are most efficient around 1,800 rpm

Cummins12V98 wrote:
"When towing, whatever gear keeps engine rpm between 1,800 and 2,000 rpm will give the best fuel mileage and power because that is the rpm window where the Cummins 6.7-liter makes peak torque.

I tow at 60 with rpm's about 1,750. Just right! That's with 4.10's. At 70 I am about 2,000.

With my last DRW I had 3.42's. I would tow in 5th at same rpm's as above, just right!