I disagree with most of the people here. I have 3.21 and average 17-17.7 MPG per tank. I drive normal, not slow but not like I’m 17 y/o either. I also live in Atlanta where traffic is hell and small portions of my commute is bumper to bumper but mostly it’s wide open interstate. I’ll settle for saying my 30 mile each way to work is a mix of city and highway.
Nonetheless, when I set the cruise at 75-80 driving to work on the interstate drinking my coffee and listening to a morning show on the radio, I see the transmission go back and forth between 7th and 8th gear on the small hills down here. Mind you, the Hemi has more than enough power to easily make it up a hill on a paved interstate at low elevation. My thoughts are that the 3.21 gears are so tall that at that speed with high wind drag and a gentle slope, the 3.92 gears wouldn’t downshift and thus, may possibly get better gas mileage than the 3.21.
This is just my theory anyway. I haven’t lifted and have stock tires on my 4X4 (Offroad package) and am 100% happy with my truck. I have another truck in my driveway right now; a 2001 Dodge 2500 4X4 Cummins. It gets marginally better fuel economy but with diesel being $1.00 at the pump per gallon and more expensive oil and fuel filter changes, I’ll drive the new gasoline truck with 3.21 gears and just drink my coffee in peace and quite while the truck downshifts. Plus, the old truck, although almost mint condition with exception to having 200k miles, is very stiff. A piece of gravel will bounce you so high that when you land, you’ll snap your spine. The new Ram is so very comfortable no matter the gears. It is a truck and not a Prius. I think it matters what you like regardless of gears. For anyone buying a new truck, today I don’t think you could do better than a Ram and I’ve recently owned all makes.