The computer only showed an average of 12.8, is that right?

That is in the ballpark for town driving or moderate towing. I would expect a bit better on the highway w/o trailer.

My 03 gets 17.8! Why would anyone want worse mileage?

03 = 5.9 12 = 6.7 6000 mile to Carlisle and back home I averaged 13.5 towing a 4x8 trailer behind my 07.

my honest dealer tech opinion of the 7-12 6.7 is that if you do not live where you can perform a "delete", save yourself the headaches and get a 13 or newer DEF truck.

Yup. When the 6.7 came out, mileage was crap. Took them a few years to figure out how to get it back.

I am in Buffalo, not sure if I can do a delete, but I sure do not want a payment on a newer vehicle that gets 5mpg worse mileage....

I will stick with old smokey for that kind of money....At least for now.

Is it just the emissions? Why would they get so much worse?

Retarded injection timing and lots of exhaust restriction would be my guess.

EGR valve, lots of soot to turn into ash = lots of regeneration time which eats fuel.

an interesting side note on the early 6.7's (7-12). Lots of people bought these truck and when used in light duty driving (no load) it wasn't uncommon for the "change oil" indicator to come on early.... like 3/3.5K miles. Well....no diesel guy is going to let some light come on and tell him to change oil, "my old Cummins I went 7k on oil changes"... so on they would drive. When the system goes into regen, there is an injection event on the exhaust stroke to heat up the cats and dpf. Of course, this process also gets fuel past the rings (thus the jet black oil on these trucks) and the ecm calculates through drive cycles etc that when the crankcase becomes 2%-3% diluted, turn the "change oil" light on. So now our oil level has become WAY overfull, hits the crankshaft, coats the cylinder walls with thin diluted oil, gets past the rings, gets burned, creates more soot, goes into regen more, used more fuel..... and the snowball from soot he77 rolls hard.

I've honestly found many of these trucks "unfixable" based solely on driving habits and use. "IF" someone bought one of these trucks, and worked the L out of it, chances are I wouldn't see them except for service.

I wouldn't believe anything the display says 100% until I divided actual gallons into miles driven. Did you hit the reset button? At least that would give you "your" average indicated, not the average of everybody that has test drove it.

That being said if your driving consists of mostly short trips and in town stop and go, the mileage is going to suck. It doesn't take a lot of fuel to keep 7500+ lbs rolling but it takes buckets of it to accelerate from a stop or from 10mph to 30mph continuously.

I just re-read the replies and I see you currently have an '03. The only way a newer truck will approach what you currently get for mileage and dependability is with an emissions equipment delete (Federal Offense) and a custom ECM tune.

Short of maybe getting one for free, I wouldn't own a diesel anything built after Oct '03.

Kevin