That depends on the specific car; the calibration of both the trip computer and the odometer has some individual variation, so YMMV.

I think the trip computer is plenty accurate for comparisons against your own past performance (i.e. if the mileage suddenly gets a lot worse, check for problems), but not so much for comparisons against other cars (and their drivers). The hand-calculated method is fairly useless for comparisons against past performance (too much 'noise' variation from tank to tank), but it can correct for the calibration errors in the instruments.

By the way, those instrument calibration errors are not unique to the NAH; all cars have them. Being off by +/- 2% is not unusual, from what I've seen on the forums.