Find the peak torque RPM for your engine, make maximum use of your tachometer and try to drive 200 to 300 RPM or more above peak torque. Don't lug the engine at or below peak torque, manually down shift to maintain RPM. The black smoke you see when you "floor it" to down shift indicates you have been lugging the engine. 2000 RPM is approximately 62 to 63 MPH on our rig, I try to keep it there on the open road and I will manually down shift on the hills to maintain speed, RPM and engine cooling. No, I don't go up the hills out west at 62 MPH but I do try to keep my hill climbing engine speed in the 2300 to 2500 RPM range.
'07 Winnebago Journey 34H, ISB-02, MH2500
Toad - '08 Ford Taurus X
Blue Ox, Aventa
US Gear UBS