Since the diameter of the tire varies with tire pressure, and all of the MPG calculations are probably based on the left front wheel speed sensor, that can explain a few MPG difference if the tire is not at whatever PSI is stated on the door jamb. That would make the on board computer virually useless. You're two pounds low, your tire is very hot, there could be 5% right there. Your tire is half worn, it probably reads even less. And where does the car pick up fuel consumed? I'm not aware of a measuring flow sensor, and we don't have MAF's, right? So I figure everyone is right.