Why do you get bad mileage on a 2004 dodge 2500 diesel?

They realy don't get as good fuel economy as older cummins, 13-16 mpg is about normal for a stock truck. If the truck has a low gear ratio that would make it worse than normal, If you keep it under 2000 rpm on the hwy it will probaby get close to 20 mpg but you will be going about 60 mph. A preformance egaust and a cold air intake should give you 2-3 mpg. Some times a chip will help out depending on your driving style, Bully Dog has a try before you buy program so you can see the difference in the truck before shelling out $500 for a chip.