How do your wind patterns change seasonally out there? I've noticed head winds, side winds, tail winds all affect my mileage, sometimes dramatically. You're in the desert, I think, so I'm guessing your fuel hasn't been winterized yet. Here in Indiana it is supposed to be Dec. 1. I'm still learning about diesel fuel quality. It's almost impossible to find out any information at the stations. One knowledgeable owner said that sulfur acts to extend storage life and the ultra low sulfur diesel doesn't last as long in the tank. I know some stations buy it from a number of different sources so the quality is bound to vary as do, I assume, different batches. I'm wondering if air temperature affects mpg, as cold air is denser. I make long flat interstate trips each week and on good days (wind cooperating and traffic) I get in the low 50s if I stay around 65 mph. I have 16k and my tank averages are around 45 mpg for the past few months. I know that will change when the cold/snow/winterized fuel arrive!