So, towing, my sweet spot is 67. When not towing, I step it up a bit. On our local roads where it is posted 55, I usually do 60 and seldom catch up to anyone. On the interstate I'll run 10-12 over the posted limit and will pass about as many vehicles as will pass me. I figure as long as someone is going faster than me, the po-po will go for them.
Now in NJ on the Turnpike, it's a different story. I'll go 80 and I'm the slowest car on the road.
2017 Lariat F-350 FX4, CC, SB, 6.7 PSD 4WD
2018 MONTY 3731FL, at our private winter site in GA