I replaced my factory muffler on my `99 F150 4x4 ext cab 5.4 V8 with a Flowmaster 40-series.
That and a K&N filter w/ a modified stock air box netted me a best of 18.7 mpg hwy and some city driving.
It sounded very nice; throaty and noticeable, but not too loud.
et conversum est retrorsum iudicium et iustitia longe stetit quia corruit in platea veritas et aequitas non potuit ingredi
"Liberty without morality is simply slavery to self." Me