i bought a flo-pro full 4" for my father for Christmas(installed cpl days before). he has an 07 classic cclb lbz. the truck is completely bone stock other than the exhaust with only 40k miles.

he went from 16-18 mpg city & 22-25 highway to 14 city and 18 highway. he's pretty upset about it. i worked at an off-road shop for a long time and installed many a diesel exhausts and never had customers complain about fuel mileage loss nor have i seen it on here.

does the truck need an aftermarket intake or the intake mod? does it need tuning??

i never woulda figured it loose any mpg much less that much. i can assure you his driving style hasn't changed, he drives like grandma!!!

any help is appreciated, if i cant fix this he wants it off

 

Does his truck have a dpf system?

 

no, its an 07 classic, same as 06 models. no dpf the exhaust is completely open/straight piped.