o4 3500 only getting 12 mpg

i am new to the Cummings game and would like to ask a few questions. I have a 04 dodge 3500 4x4 with 4.10 gears and 48re trans. it is stock now other that the K&N air intake , but i think it still has some program left in it. it is alot faster than any other factory truck comparable to it. i only have been able to get 12 mpg from it since i purchased it. everyone around here that has trucks like it are getting 16 to 21 mpg, so what can i do to get that out of mine. any help would be appreciated.  

Factory wheels & tires on it, or do you have a lift?  

i have 325/60/18 tires on 18 x 9.5 wheels, factory called for 265/70/17. as far as a lift the dealer said it had a 2 in. lift but from what i can see there is all factory springs and the  suspention up front looks to be un touched. this truck came from an auto sale to the dealer from which i bought it from , and they have'nt been able to give me any definate answers about the truck. i do know that  it had  stacks and some type of programer and had a pyrometer and a temp sensor or something hooked to the trans. wish i could find the original owner so i could get the scoop on it.  

Its cummins not cummings  

like i said im new to CUMMINS so forgive me and hope you continue to correct me on my mistakes ,otherwise how would we learn if not by the  mistakes we make  

I am unfamiliar with the new cummins engines, but I know that on the 6.0 powerstroke there is a mass airflow sensor in the intake tube. When running a aftermarket filter such as a K&N, if the filter element is not cleaned and oiled properly it can allow dirt to pass through and biuld up on the sensor. I the case of the 6.0, it totally tanks your mpg. This may be something to look into, since you don't know the history of the truck and how it was taken care of. I do believe that there is a cleaner you could pick up from your local auto parts store.
If I'm wrong or off base, I'm sure someone out there will correct me.  

Its gotta be the 4.10 gears, does all your buds have 4.10s that are getting 16 to 21? One of mine does he also has 4.10s and its a 1 ton I think an 05 and he just traded it off bc it only got 12mpg too.  

is that hand calculated, also those seem to be way larger tires so they will also drag the mileage down  
I burn more diesel and run more miles in a week the most of you people do in a month. ctds that have been used 97 3500, 98.5 3500, 98.5 3500,01 3500, 03 3500. those that are being used 06 3500, 07 3500. and yea these trucks earn their living

thanks for that info and i did buy the cleaning kit and used it recently. it seemed to help my power and mpg but only up to 13 mpg. i will go and check the mass air flow sensor to see if that works . i also am going to check that on my 06 f 550 work truck is sucks in the milage dept. 6 to 8 mpg  

yeah three of them do and they range from99 to 06 models the 99 gets 16 intown and 19 over the road and its bone stock and automatic. i have been talking to the local dealer mechanic and he says i need to flash the cpu and install the factory settings with all the updates. i plan to do that then buy a smarty programer with economy and try that. i also am saving to buy taller tires and rims to get a different gear ratio,all that is work in progress though.  times are tough with money now so toys are only good if you dont have yto worry about a house payment.  

pbrbullhauler that is hand calculated and i agree about the tires i dont like them that wide and the rims have a offset that putts them way outside the fenders. i would love to find a set of factory rims that are 18 to 20 inch. or put the 10 lug converson on it and run 22.5. the tires would last longer and be tougher than what i have now.  

try the reflash plus check your oil to make sure that diesel is not getting dumped in it. the injectors could have have gone bad and they sometimes will put diesel in the oil  
I burn more diesel and run more miles in a week the most of you people do in a month. ctds that have been used 97 3500, 98.5 3500, 98.5 3500,01 3500, 03 3500. those that are being used 06 3500, 07 3500. and yea these trucks earn their living

yep the reflash is my next step and the oil is fine. i do have some black smoke at 45 to 55 mph and i stomp the throttle, and the pickup is phenominal. i feel that the computer still has a hot program and its dumping fuel in to compensate, and thats why my fuel milage is so bad. now how do you figure milage other than gallons divided by miles.  

I know with ours we have to add 7 miles to every 100 miles because the larger tires offset it that much. We went on a trip to michigan and our buddy had a 09 and his trip meter had 28 miles more than ours( on a 400 mile trip) and he was factory set up also we read that somewhere it was like 3.5 miles for every tire size bigger you go i believe.  
2005 2500 dodge ram banks 6 gun w/speed loader,turbo back stright pipe, S&B intake on its way, phantom gauges,20" gear wheels with Cooper Xenon tires,and much more to come

well the computer had to have  been recalibrated, because a cop friend of mine has checked it with his radar and i have checked aginst others a cruising speed and it is right on.