Diesel Truck MPG

A good source to compare mpg is fuelly.com. The data is self-reported by vehicle owners, and it typically covers hundreds of vehicles and hundreds of thousands of miles. Even accounting for some bias and inaccuracy in the numbers, the volume of reports would tend to flatten out the inconsistencies and give a reasonably accurate picture of real-world fuel efficiency.

I did a quick comparison of Chevy, Ford & Dodge diesel across all reported model years. If you look at the bar graphs at the top of each page linked below, you'll see that Dodge seems to have the edge. The average mpg "curve" is pretty flat between 15-17 mpg, whereas Ford and Chevy drop off sharply after 16 mpg (i.e. much fewer vehicles getting better than 16 mpg). A small difference, but as I said, averaged across all vehicles and all kinds of driving, it does reveal a trend.

You can also narrow down your comparison to look at specific model years.

Dodge:
http://www.fuelly.com/car/dodge/ram%202500/diesel%20l6/truck
Ford
http://www.fuelly.com/car/ford/f-250%20super%20duty/diesel%20v8/truck
Chevy
http://www.fuelly.com/car/chevrolet/silverado%202500%20hd/diesel%20v8