I get this with the girlfriend all the time. She's always 'telling' me that I'm in the wrong gear and that I should be in 3rd gear. (this is in a 5spd 2.5ltr straight-six Z4, just shy of 2k rpm in 4th). I can't get across the point that I'm not accelerating, I'm not loading the engine, it's just rolling along. She doesn't understand why I might change down a gear when overtaking on the motorway either.
It's quite annoying because she scorns that I'm damaging her car, and "how would you know, you drive an auto".
She's been driving about a year.. She doesn't understand what RPMs and gears actually mean, all she knows is that when she asked someone what gear you should be in at a given speed, they told her a number.