As a note: the difference is roughly 220 RPM on my truck between 3:73 & 4:11
The final drive in the transmission is .67 and tire size is 31.5" it's just math...
The 2015 6.7 diesel in my truck gets the most efficient power between 1800 & 2000 RPM more than 2000 and the fuel consumption goes up quickly for little gain in speed.
I generally set my cruise at 1800-1850 which is roughly 67-69 mph