[mpg_numbers_guy]

In terms of economy accelerating at low RPM + high load is best, but I'm not sure how a turbo affects that.

I always thought shifting at 2K RPM or lower was best for diesels anyway because of the torque provided? Most semis have a 3K redline from what I've read.

In most cars lower RPM has a deeper sound under heavier loads (more noticeable with an exhaust leak) but that doesn't mean it's being lugged. You should be able to feel it when you're lugging the car by the car jerking slightly or hesitating when being lugged at very low RPMs.