This is pretty subjective territory. Considering my driving habits and the local terrain, I will never crack 20mpg on any full size diesel ever sold in the U.S. market. The only ones I've heard of getting close to that were single rear wheel, 4x2 2 door cab manual trans vehicles. The new 6 and 8 speed auto trans equipped trucks allegedly can get better fuel economy than the manual trans trucks.
Expect 15mpg on all of them unless you're a "hypermiler".