i try to drive the sweet spot for my set up, regardless of the way the rest are driving. That sweet spot moves around though. On the flat with no head wind I like 64 mph. If driving into a headwind or uphill i'll just follow the sweetness and drop to around 58 mph. It all has to do with throttle position resistance; something you feel.
The woe is that my speed is usually faster than the trucks in the right lane and slower than the cars not in the right lane. So, I find myself weaving in and out of the slow lane more than I would like.
Peak torque on a gen2 Cummins H.O. engine is 1600 rpm, 55 mph on the speedo with stock tires, but the sweet spot for this particular engine is around 1850-1950 rpm.
If I'm in a hurry I'll drive 70-75 and stay up with left lane traffic with a slight deterioration of mpg. We just got back from a 1600 mile road trip in the TC, and averaged 12-13 mpg. Several times we paid less than $3 per gallon for No. 2, but it averaged around $3 per. $2.59-3.09 per gallon was the range.
Our traveling weight for this trip was 10,400 pounds. No trailer. No jacks. We were out 5 days and overnighted in sundry locations both in Seattle and rural spots in OR.
jefe