I understand gear ratios and tire diameters perfectly. What I don't understand is why there is as much difference in fuel economy between the 3.31 trucks with 18" rubber and the 3.55 trucks with 20" rubber as there is. But from experience I know there is a big difference. As well because the owners manual states 20% less mileage per tank of def under normal driving conditions is expected with the 3.55 axle vs the 3.31 I will state again that I won't be surprised if something is different in the fueling softeare between the different axle ratio trucks.