My quad cab 2wd with 3.55 that I drive about 99% city averages 16-18mpg. When we do highway travel (camping, travelling out of state, etc...) 20mpg at 70-75mph is easy and 21-23mpg at 65-70mph is doable.
I have a 4x4 4.7l with 3.92's (upgraded from 3.55), my fuel economy change was negligible between the 2 gearsets. Consistently 14 mpg or less in town if I have a downtown type environment where the speed limit never exceeds 30 or so and stoplights every coule hundred yards. regular In town driving nets 16-18, hwy is 20-24.
one thing about the 4.7 though, it is a strong engine, but the Ram is a heavy truck...if you get a headwind on the highway you will be drinkin' fuel. I have gotten as little as 15mpg hwy, if I get a nasty headwind.