See you've got a lot of variables in these trucks and therefore a lot of mileage differences. 30 mpg is probably not far fetched depending on the speeds driven. Low speeds, say around 60 or so, may give you high numbers. 2wd or 4x4? Big difference. The biggest is if some of these folks are claimining their MPG based on the overhead computer, especially if they combine that with a tuner (Banks sucks, if you want real power go with something else.)
For reference my 14 year old '93 Cummins, factory stock with nothig but a little tweak on the smoke screw and Lucas synthetic in teh pumpkin and Getrag, gets a wide range of mileages. If I run 65 on the interstate I get 20 mpg. Bump it up to 70 and that drops to 19.5. Go up to 80 (which is about all it will do) and it drops to about 18. Going the other way, if I run 55 I can get into the low to mid 20s.
For the new folks here, it would be nice if ya'll filled out the signature box with your truck specs so that we know what you're talking about when you post a question.