I have two 300tdi's: one in the D1 and one in the RRC.
My D1 has been built up. Lockers, sliders, armor, bumpers, winch, roof rack, etc. I get 20 on the highway if I'm lucky.
My wife's LWB gets about 26 on freeway. It runs better than my 300. The same guy did both installs. He is a diesel...