You can't expect real milage until you have some miles on the truck. I don't think you can say it is even broken in until you have 40 to 50K on it.
I own the 5.9 Cummins. I am glad I bought one over the 6.7. For all the other guys touting the other brands, all I can say is look into the size of the rods and pistons on the Cummins. Nuff said. Them are some beefy parts.

 

I now have 4500 miles on the truck. Almost all of the driving with this truck is with a trailer hooked on the back, so I figure it will break in sooner than 50,000 miles. Still, with the limited amount of miles I put on this truck, I figure it will be about six years before the engine breaks in. My fuel mileage pulling the toy hauler in the pitures earlier in this thread has risen to between 8 an 9 mpg, which is exactly the same as the two prior diesel trucks I towed this trailer with. It just plain takes some fuel to move that trailer down the highway.

4,500 miles later I still say this is the best pulling truck I've ever owned.

 

JUst don't expect it to be a rocket ship. It is a Diesel after all... but I'll tell you, with mine at least, the heavier the trailer, the harder it pulls and it LOVES more weight. I think it is because the Turbo is in boost longer maybe. You look at the slugs in your typical Cummins, and they are the size of 3 lb coffee cans. The RODS are like comparing your thigh to your forarm when comparing to some of the competition.They are stout.

 

The truck has just over 6000 miles on it now. I recently had the chance to tow a 4 door Jeep Wrangler on an auto transport trailer from Grand Junction, CO to home on I-70. This 6.7 Cummins is an animal. 80 mph up and over the steepest grades on I-70, while towing a Jeep on a trailer, is impressive. I've never been able to do that before with other tow rigs (Duramax, Cummins 5.9, Dodge V-10, etc.) This truck just flew past other drivers on the uphill grades. What a beast! Average fuel mileage for the entire 800 mile trip was 10.8 mpg. This truck just keeps getting better.

 

dodge!! my dad had a 2001 dodge 3500 5.9 cummins with 200,00 miles on it and worked like a champ. he towed a 53 foot 4 car hauler that weighed approximently 16,500 lbs for our bissness and and towed it like it was nothing just wished he keep it great running truck just keep up with them and they will last for ever!! but we started buying trucks and needed more braking power so we got a tractor trailer but great running truck!! i would reemnd it for any one!!