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.
"That's a big bite of crap sandwich, my friend..."
"Alcohol doesn't solve any problems, but then again, neither does milk"