They are lying.....In stock form I have never gotten over 23mpgs in my Jeep CRD....it generally stayed in the low 20s for hwy only. Now there are some tunes out there they can give you 1-2 mpgs...no one is getting 30mpg in a CRD WK...even 2WD CRDs with tunes are getting only 26mpg..and that is with a rear gear swap.
Aero, full time 4wd, and the transmisson (5speed) are all limiting factors.
Just as a comparison so you do not think I am making this up....I also have a 2010 Mercedes ML350 CRD as well same engine as the Jeep CRD...it has much better aero, better transmission (7speed) and a different all wheel drive system....I have 55,000 miles on the ML350 and I get 26-28 on a regular basis with a mix of highway and city.
So 19mpgs with Duratracs, 2.5 inch lift, winter fuel, driving 75mph....I am cool with 19mpgs. That gives me close to 400 miles....I will be adding a GDE tune soon to bump me back over 20mpg.....
Where the diesel really shines is using the 400lb ft of torque in low gear to crawl over anything on the trail.