Just finished a 5282 mile round trip to Yellowstone Nat'l Park, but am dissappointed in my mileage. I was hoping to finally break 40 mpg, but fell short. My Q5 ran perfectly with excellent comfort and road manners, and we were loaded and running heavy, but I was sure I could break the magic 40 mpg barrier. Kept the speeds generally below 75 (except during a Texas rain storm avoidance - quick sprint to 95), and drove like a "mature" adult that I am. No matter, the best I could do was 38 plus mpg. Had rain, winds, snow, tornado, traffic, mountain climbs (9114 ft), and all the other good stuff you find on a long trip (avg 500 miles per day)....... but no 40 mpg. Guess I'm a bad driver!
How many miles are you on now? Diesels seem to take longer to break in. Cruising speed in important, but so is consistency, try sticking to a set speed for long intervals. Assuming you had ac on too? That can use 5 to 10% also.