landspeed -
I think of it as being similar to baseball's "
Mendoza Line", i.e. every car has a "Lug Line". For me, that definition is the point where :
function(MPH, Transmission Gear) = engine RPM below the normal idle RPM.
The above assumes your are on the flat and no one is charging up your butt. At this point, if you are a feather-light on the throttle, you can accelerate gently as needed.
Lugging is not efficient from an engine POV, but I think that in many cases, the
lower RPM X @ MPH Y usually beats the "engine operating at peak efficiency" argument.
If you are driving at the "Lug Line", external conditions may change (hill, need to speed up) such that you will have to drop into a lower gear.
CarloSW2