Sometimes I wonder about how they come up with these mileage ratings. I know I was told by a dealer one time that the ratings are usually a little lower overall from real world mileage as a marketing tool/feel good kind of thing. Car companies would rather have the customer pleasantly surprised at the slightly better than posted mileage than angry customers complaining that they get 1 mpg less than what's posted on the window sticker. Plus got to take in account that no 2 people driving the same car will get the same mileage.
As for instance, years back my brother had a new '99 chevy 1/2 ton standard cab 2wd pickup. 4.8 liter auto. Constantly going north with snow machine in the back, getting 25-28mpg freeway driving. Figuring in short trips, city driving, dirt roads, etc. it's overall mileage was more in the 16-18 mpg rating. So I'm gonna assume that with the right owner running the freeways could probably get that new diesel closer to 30 mpg than one would think.