I use 6th on the highway or any reasonably level (i.e. not mountainous) stretch where I would use cruise control (assuming the traffic situation is ok).
Just drove to Vermont on Saturday.
215 miles total NJ->VT; 125 on I-87, then 80 miles on 2 lane state roads with one 22 mile semi-limited access road in the middle.
75 MPH on I-87 with cruise control; 63 MPH overall for the entire trip; averaged 23.5 MPG.