Strong fumes from exhaust

What is your question?

I smell strong fumes from the qexhaust pipe. Also I recently noticed extreme drop in gas mileage from 300+ miles to empty on a full tank of gas, down to a little over 200 on a full tank of gas. A little sluggish but no hesitation. No leaks, hard rattling noise on initial startup. Are these issues related and What can this be?

What makes this problem better or worse?

No differences

How long has your 2006 Lincoln Navigator had this problem?

3 months after replacing rear 2 spark plugs