I checked with a few customers on Friday, one has a 2004 2500 with an automatic, he says he gets around 17 mpg unloaded, down to 12 loaded. The other guy says he's getting 15 unloaded and 10 loaded. Didn't ask which trans he had.
The trainer told me that when Dodge started upping the hp with the common rail injection system, that's when they lost some of the fuel economy. I'm not saying the CRD part caused it, but it's when Dodge/Cummins was touting their 610 ft/lb torque ratings. To get more hp in a diesel, ya gotta turn up the fuel. When you turn up the fuel, your economy goes down. Generally speaking anyways.
I've got a hint for some Cummins owners. If your mileage has been dropping, one thing you can easily check are the hose clamps on the hoses to the intercooler. I've seen quite a few lately that are loose. Loss of boost = loss of power, loss of fuel economy too. There's a special fixture that we can attach to the hose off the turbo, and we can check for leaks. We've checked max boost before and after tightening the clamps, we've seen as much as a 8# boost increase before/after just by tightening the loose connections. I think my tech said for every 2# of boost lost, it might equal 1 mpg decrease in fuel economy. Then again, I'm probably a little off with those numbers. But it's still a common problem, easy fix.