I don't know the technical definition of lugging; this non-tech
definition matches what I thought it was though: engine jerking/shuddering. The kind that makes you go, "oooohhh, that can't be good..."
Treb: sounds like you may be after a metric that I wish government agencies would provide: just show me the fuel consumption across a range of constant speeds in top gear. Forget trying to simulate an entire trip to grandma's house for freaking sake!
EDIT: deleted some other rambling. Need to get some sleep. :P