yeppp it was complete garbage. Even had it to the dealer a few times... they said it was my "aggressive" good year duratracs I just laughed. I averaged 12-13MPG highway empty and 9-11 towing depending on the situation. So I deleted it and it sky rocketed immediately to a consistant 19 to 21.6 best mpg. and 12-13mpg towing our rig. I loved that duramax deleted, BUT then new far more strict legislation came in and long story short I would've had to put all the emissions equipment back on in order to pass safeties, said I wouldn't own that truck with the emissions equip back on, so onto the RAM we went.
it had a bigger tank at basically 36 gallons and seen on average 600-650KM per tank on the highway at 110km/h tank as per the computer which was surprisingly close. Ram has a 32 gallon tank, and see on average 830km per tank on the highway at 110km/h. So still not amazing deleted mileage, but quite a bit better than the GM.