I have a 2500HD with an LLY and have had nothing but problems since day one. The truck was purchased in March and has since been in the shop multiple times. The first was for injectors and the last time which was a couple of weeks ago was for the ECM. Ever since the ECM was replaced/reflashed my gas mileage is HORRIBLE. I am lucky to get 14 in the city, and towing a 5000lb trailer somewhere around 8. Any info would be appreciated, I know there have been multiple threads regarding this issue in the past. Or does anybody know of a good diesel dealership in the Detroit area?

14 in the city doesn't sound unreasonable. 8 towing a trailer seems low. I get 13 towing a 7 x 14 ' 5000 lb cargo trailer at 65-70, 12 or so towing a 8000 lb 8.5'x20' cargo trailer at the same speeds.

mileage is hand calculated, not from the DIC.

Take it back to the dealership and let them know what it's doing.

yea, those numbers sound fairly accurate.

one question...why dont the DIC's calculate milage very well. i had a 6.0 gas last year and it was always 99% right but on my duramax its way off, it will say 19 on highway and im only gettin like 14. its also wierd when i fill up the fuel used will say ex. 14.7 gallons used and i will put like 24 in, now c'mon thats way off. you think they could get it closer than that

p.s. my lly only gets abou 10 or 11 in town and id say pulling an empty skidloader trailer (about 2000 lbs.) on the highway is gettin about 12

sounds just about right..I get almost 15 in the city and even better highway. Its getting better with each tank though. I wouldn't expect a brand new Diesel to come out and get 20mpg before its broken in and what not.

Your DIC will be off with any tuner/chip, the readings are optimistic because the algorithm is thrown way off when the fuel tables are changed...

one question...why dont the DIC's calculate milage very well. i had a 6.0 gas last year and it was always 99% right but on my duramax its way off, it will say 19 on highway and im only gettin like 14. its also wierd when i fill up the fuel used will say ex. 14.7 gallons used and i will put like 24 in, now c'mon thats way off. you think they could get it closer than that

p.s. my lly only gets abou 10 or 11 in town and id say pulling an empty skidloader trailer (about 2000 lbs.) on the highway is gettin about 12

very logical answer thanks