I'm not sure if I'd expect that to be honest. The FACT is that the diesel motor will get better fuel economy and a big gasser. The claims of 19-25 mpg I find quite hard to duplicate with either my 7.3L Powerstroke or my "mileage" 2004 Dodge. They are both better mileage than their gasser counterparts by about 30-50% in my estimation. You should be able to get that mileage on the freeway at or just below 2K rpm without any starts/stops or hills (what I'd deem optimum conditions).
On BOTH trucks, the onboard computer estimation was what the other people claim as actual mpg. Mine has been about 10-20% less when hand calc'd.