Ok, a 3.5L should not be getting 16MPG, and with a rotten egg smell out the exhaust, its running rich.
Have you ever followed another car on a flat stretch and then started going up hill and all of a sudden, you smell the same smell your smelling? That'* because the computer increased the amount of fuel going to the engine to compensate for the hill. Same thing seems to be happening here in your case, but its constant. This is why your MPG sucks.
Are you sure your fuel pressure regulator is working correctly?