Diesel Bombers - 6.7 Mileage and Dpf problems

My mom recently bought a 2012 1 ton with 6 inch lift and 35's. she has done a cold air intake, 4 inch dpf back exhaust and an edge juice w/attitude. The truck will not get better then 25L per 100 km in town and not better then 21L per 100 km highway, shes not driving agressive or doing anything incorrect that i can see, also the Dpf regen kicks on at least once a day (she only drives around 40km per day on average) she does not want to do any dpf or egr deletes yet because the truck is only at 9000km and she was told it would void her warrenty. The "Cummins specialist" at the local dodge dealer told her to drive her truck in 4th or 5th gear at highway speeds (100km) a couple times a month to force a regen and clear it out, and that should be the only time it would kick on.
Her boyfriend has a 2012 1 ton with no lift and stock tires with the same cold air, 5 inch dpf back exhaust and the same edge juice w/attitude, and a transmission upgrade (i dont know what this was, he took it to a transmission shop and said they did a plug&play shift kit?) his truck gets almost 20L per 100km in town and 15L per 100km. also his rarely goes into regen. this truck is at 10000 km.

i was wondering what the issue with the first truck could be? would the oversize tires cause that severe of a milage drop? or maybe the transmission having an issue? Or a Dpf pressure switch faulting? Any help would be appreciated!