the 6.7 diesels get horrible mileage due to DPF in exhaust and EGR plugging up the intake and the VGT vanes.
5.9 common rail should be getting 17-24...if yours is not you may have a cracked injector tip.
might want to check out the injectors (they are notorious for cracking tips on the common rail engines)
also once you get that done, get a smarty programmer and put in the mileage tune with high timing. it will increase fuel economy a bit and reduce overall EGTs.
if you want lots of power to go with it, you can go with a smarty, or the now more popular EFILive.
as for economy: I get 14, but my injectors suck. they are smokey as all hell and haze at idle. I need to have them pop tested or just replace them as I believe the pop off pressures are all over the place on them and definitely set way too low...probably at 260bar when they should be at 310bar.