315s look good under a Super Duty.
Even better, they can get you around off road.
...but you notice it at the pump. $$$:Wow1:$$$ A Super Duty has a great payload capability, but they are heavy, even empty.
I got a 7.3 and I'm re-gearing from 3.73 to 4.30 with air lockers this summer. With a gasser and a 35" tall tire, you'll want 4.56 gears. You just have to weigh the cost v benefit. My truck is a daily driver, but I'm retired Army, I don't have to drive an hour each way to work every day. I'll pay the fuel cost if it gets me that off road capability. I let my wife have her "Girls' night out"... she tolerates my big diesel "man-cave".
Everybody happy, happy, happy.
Now if I can just talk her into a winch... (that I said I wouldn't need with my lockers...)