Like for like, the diesel only get's 2mpg more unloaded IME. Add a lift and 38" tires, that might be 3-4mpg.
The #'s only count if you quantify the exact conditions. For example:
55mph cruise control, perfectly flat ground, no stops, XL OEM wheels tires, 6.2L: 19-20mpg.
City #'s are...