I switched to 35x12.50-17 Toyo MT's last summer on my '04 CTD and I love how they perform in the snow/ice. Way better than the BFG A/T's that were on it before. But back to the point of this thread:
My truck weighs right around 9,000 lbs. (Quad cab longbed) according to the scales I have been on. The lowest mileage I have ever gotten on a tank was 11.3 mpg - towing my 11,000 lb. 5th wheel. I have kept my mileages in an excel spreadsheet since I bought the truck about a year ago and I consistently get 13 - 15.5 mpg whether all stop-and-go or freeway. That is with 35" tires, stock 3.73 gears, and auto tranny. Your mileage should at least be similar even though yours is a PSD. I would say that something is up with your injectors or something also.