This is a link to the dieselpage and the mileage on the 2004.5 duramax diesel
http://www.thedieselpage.com/mileage.htm
You will notice that I am from Canada, our gallons are larger than yours, hence the 25 mpg. I used google to search for "duramax diesel mileage", the above link was what I found over a year ago. Motor Trend magazine ran an article in their Jan/Feb 2009 issue that shows a 2008 Chevrolet Silverado 2500HD with the diesel, (The article I am pointing to is on page 34) the driver lives in Sante-Therese, Quebec, Canada and he gets 24 mpg when not towing and 13 when he does. The picture shows him towing a dual axle 5th wheel trailer. All I am saying is that 20 plus mpg is possible. All you have to do is search.