[Big Dave]

Hard lugging is rough on an engine because it sets up a resonant torsional vibration throughout the entire drive train. You’ll rapidly find the weak link. BTW don’t overbuild that weak link. It was put there deliberately to fail before more expensive components. A lot of lifted 4x4 guys oversized their U-joints when they failed only to find that axles and transfer cases broke. Beaucoup expensive.

But (at least for turbodiesels) the sweet efficiency spot is at a high load/low RPM just above a lug. Lugging occurs in arrange of load and RPM. It is possible to lug an engine turning 14,000 RPM if you load it up too much. If your engine is lugging, try reducing load (pull your foot out of it).

My F-350 (7,000 lb empty, International T444E engine (mostly stock), ZF 6-speed, Gear Vendor overdrive, 3.08:1 axle) rolls down the highway at 50 MPH at 950 RPM. That is juuuuust over a lug. But at 70 MPH and 1325 RPM lugging is nowhere in sight despite the fact my aero drag HP went up by 175%.

The strategy works well for diesels as they operate at excess air. Gas engines are married to a narrow band of fuel-air mixture ratios. Slowing the engine down reduces the pumping loss and you really need not speed up until she smokes.