Why is my 2001 dodge 3500 diesel auto. only getting 13 mpg empty? Seems

[RNLD66]

to run fine but bad gas mileage. Why? I have a 2001 Dodge 3500 dually truck. It is two wheel drive, with a diesel engine and an automatic transmission and 3.55 gears in the rear end. I bought the truck in the summer of 2007 with 48k miles. It now has 71K miles. The truck runs and drives great but it has always gotten poor fuel mileage. I get about 13 mpg on the highway without a load. If I am pulling something, my mileage drops to about 8mpg. Everyone I have talked to with the exact same truck says that they get about 20 to 21 mpg. My buddie's 2003 dodge diesel gets 21mpg. I have changed all the filters with no results. The dealership says it checks out fine on the computer. What could be wrong with it to give me such poor mileage?

[PaulN]

do you have hwy gears or working gears I put a chip in mine it helped also try a K&N air filter