Cummins website says it takes anywhere from 5000 to 20,000 miles to break in... they state that if you baby it, that will delay the break in period up to 20,000 miles.

 

First off help us out and tell us about your truck in your Signature.

That well help you in the future. Tow something with it something with weight it will thank you. When I drove mine off the lot I didn't get but about 13-14 mpg. It took about 30,000 miles for mine to start getting anything descent in mileage. I suspect some engines might need alittle more time to breakin. Just like women ain't all the same. 42,000 and some change on the OD and I get anywhere from 17 to 20 mpg at 65 mph no load and not much wind and flat terrain. I get to 70 mph empty and it gets about 16 mpg. I also run Power service in it about 12 ounces for a tankfull that helps alot. Just got back from a round trip from home to GunBarrell city about 600 miles got 17 mpg. empty.

Sittenbull

 

Overhead showing 19-20mpg, hand is 14.8; 75%cruise/25% set&go. (No Loads). No smoke/easy shifts/quiet.
Told dealer not happy with mileage, he reflashed at 5k miles. Ran out tank. Noted slight smoke at start, some smoke at accel./more RPM drop at shifts/louder/doggier.
Last tank drove at 55, shifts at 18-1900, overhead shows 19.2/Hand shows 16.3.
Good truck, but not what I was lead to believe. It was bought for occassional long range interstate hauls running close to max. weight. If I keep it will probably install gearvendors O.D. unit. (Warranty Issue?). It does look as if it is in the same ballpark as the rest.
Great board, lots of good info.

 

I know trucks will vary in the mileage they get, but something does seem a bit off with yours. Mine gets about 17 mpg unloaded at around 67 mph on the cruise control. It's an auto, dually, 4x4 quadcab longbed with 3.73's. Hard to weigh much more, in other words. It only has 2500 miles on it, so I think it will improve some more. I don't know what it would get at 55 mph, but I'd imagine it would approach 19 mpg. By the way, the 17 mpg is both on the overhead and hand calculated, so it's not just a faulty reading. If I were you, I might press a bit more to see if something is a little off with the truck.

 

After the re-flash and a 3 week trip with a heavy slide in camper ( I think the engine is starting to break in now at 9k) I have been averaging around 18. Although I did have one tank at a little over 20. The back country roads around my house are 45-50 mph, so I think this has helped. I keep the tires full and use neutral to coast on the downhills.

It is hard to calc each tank as it takes great patience to completely fill the tank to the same visable spot each time.

If your truck is in the date range for the re-flash I would highly reccommend it. I picked up about 2 mpg.

Hope this helps.